Memblock is one of the methods of managing memory regions during the early bootstrap period while the usual kernel memory allocators are not up and
running yet. Previously it was called `Logical Memory Block`, but with the [patch](https://lkml.org/lkml/2010/7/13/68) by Yinghai Lu, it was renamed to the `memblock`. As Linux kernel for `x86_64` architecture uses this method. We already met `memblock` in the [Last preparations before the kernel entry point](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html) part. And now time to get acquainted with it closer. We will see how it is implemented.
running yet. Previously it was called `Logical Memory Block`, but with the [patch](https://lkml.org/lkml/2010/7/13/68) by Yinghai Lu, it was renamed to the `memblock`. As Linux kernel for `x86_64` architecture uses this method. We already met `memblock` in the [Last preparations before the kernel entry point](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html) part. And now it's time to get acquainted with it closer. We will see how it is implemented.
We will start to learn `memblock` from the data structures. Definitions of the all data structures can be found in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file.
@ -85,7 +85,7 @@ These three structures: `memblock`, `memblock_type` and `memblock_region` are ma
As all API of the `memblock` described in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file, all implementation of these function is in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c) source code file. Let's look at the top of the source code file and we will see the initialization of the `memblock` structure:
As all API of the `memblock`are described in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file, all implementation of these function is in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c) source code file. Let's look at the top of the source code file and we will see the initialization of the `memblock` structure:
Here we can see initialization of the `memblock` structure which has the same name as structure - `memblock`. First of all note on`__initdata_memblock`. Defenition of this macro looks like:
Here we can see initialization of the `memblock` structure which has the same name as structure - `memblock`. First of all note the`__initdata_memblock`. Defenition of this macro looks like:
```C
#ifdef CONFIG_ARCH_DISCARD_MEMBLOCK
@ -137,7 +137,7 @@ Every array contains 128 memory regions. We can see it in the `INIT_MEMBLOCK_REG
#define INIT_MEMBLOCK_REGIONS 128
```
Note that all arrays are also defined with the `__initdata_memblock` macro which we already saw in the `memblock` strucutre initialization (read above if you've forgot).
Note that all arrays are also defined with the `__initdata_memblock` macro which we already saw in the `memblock` structure initialization (read above if you've forgotten).
The last two fields describe that `bottom_up` allocation is disabled and the limit of the current Memblock is:
@ -147,12 +147,12 @@ The last two fields describe that `bottom_up` allocation is disabled and the lim
which is `0xffffffffffffffff`.
On this step initialization of the `memblock` structure finished and we can look on the Memblock API.
On this step the initialization of the `memblock` structure has been finished and we can look on the Memblock API.
Ok we have finished with initilization of the `memblock` structure and now we can look on the Memblock API and its implementation. As I said above, all implementation of the `memblock` presented in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c). To understand how `memblock` works and is implemented, let's look at its usage first of all. There are a couple of [places](http://lxr.free-electrons.com/ident?i=memblock) in the linux kernel where memblock is used. For example let's take `memblock_x86_fill` function from the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c#L1061). This function goes through the memory map provided by the [e820](http://en.wikipedia.org/wiki/E820) and adds memory regions reserved by the kernel to the `memblock` with the `memblock_add` function. As we met `memblock_add` function first, let's start from it.
Ok we have finished with initilization of the `memblock` structure and now we can look on the Memblock API and its implementation. As I said above, all implementation of the `memblock`is presented in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c). To understand how `memblock` works and how it is implemented, let's look at its usage first. There are a couple of [places](http://lxr.free-electrons.com/ident?i=memblock) in the linux kernel where memblock is used. For example let's take `memblock_x86_fill` function from the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c#L1061). This function goes through the memory map provided by the [e820](http://en.wikipedia.org/wiki/E820) and adds memory regions reserved by the kernel to the `memblock` with the `memblock_add` function. As we met `memblock_add` function first, let's start from it.
This function takes physical base address and size of the memory region and adds it to the `memblock`. `memblock_add` function does not do anything special in its body, but just calls:
@ -160,7 +160,7 @@ This function takes physical base address and size of the memory region and adds
function. We pass memory block type - `memory`, physical base address and size of the memory region, maximum number of nodes which are zero if `CONFIG_NODES_SHIFT` is not set in the configuration file or `CONFIG_NODES_SHIFT` if it is set, and flags. The `memblock_add_range` function adds new memory region to the memory block. It starts by checking the size of the given region and if it is zero it just returns. After this, `memblock_add_range` checks for existence of the memory regions in the `memblock` structure with the given `memblock_type`. If there are no memory regions, we just fill new `memory_region` with the given values and return (we already saw the implementation of this in the [First touch of the linux kernel memory manager framework](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html)). If `memblock_type` is not empty, we start to add new memory region to the `memblock` with the given `memblock_type`.
function. We pass memory block type - `memory`, physical base address and size of the memory region, maximum number of nodes which is 1 if `CONFIG_NODES_SHIFT` is not set in the configuration file or `1 << CONFIG_NODES_SHIFT` if it is set, and flags. The `memblock_add_range` function adds new memory region to the memory block. It starts by checking the size of the given region and if it is zero it just returns. After this, `memblock_add_range` checks for existence of the memory regions in the `memblock` structure with the given `memblock_type`. If there are no memory regions, we just fill new `memory_region` with the given values and return (we already saw the implementation of this in the [First touch of the linux kernel memory manager framework](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html)). If `memblock_type` is not empty, we start to add new memory region to the `memblock` with the given `memblock_type`.
First of all we get the end of the memory region with the:
@ -235,7 +235,7 @@ and copies memory area with `memmove`:
After this fills `memblock_region` fields of the new memory region base, size and etc... and increase size of the `memblock_type`. In the end of the execution, `memblock_add_range` calls `memblock_merge_regions` which merges neighboring compatible regions in the second step.
After this fills `memblock_region` fields of the new memory region base, size, etc. and increases size of the `memblock_type`. In the end of the execution, `memblock_add_range` calls `memblock_merge_regions` which merges neighboring compatible regions in the second step.
In the second case the new memory region can overlap already stored regions. For example we already have `region1` in the `memblock`:
@ -301,7 +301,7 @@ If none of these conditions are not true, we update the size of the first region
this->size += next->size;
```
As we update the size of the first memory region with the size of the next memory region, we copy every (in the loop) memory region which is after the current (`this`) memory region to the one index ago with the `memmove` function:
As we update the size of the first memory region with the size of the next memory region, we move all memory regions which are after the (`next`) memory region one index backward with the `memmove` function:
```C
memmove(next, next + 1, (type->cnt - (i + 2)) * sizeof(*next));
@ -330,7 +330,7 @@ That's all. This is the whole principle of the work of the `memblock_add_range`
There is also `memblock_reserve` function which does the same as `memblock_add`, but only with one difference. It stores `memblock_type.reserved` in the memblock instead of `memblock_type.memory`.
Of course this is not the full API. Memblock provides an API for not only adding `memory` and `reserved` memory regions, but also:
Of course this is not the full API. Memblock provides APIs for not only adding `memory` and `reserved` memory regions, but also:
* memblock_remove - removes memory region from memblock;
* memblock_find_in_range - finds free area in given range;
As you can see `level2_fixmap_pgt` is right after the `level2_kernel_pgt` which is kernel code+data+bss. Every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses` enum from the [arch/x86/include/asm/fixmap.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/fixmap.h). For example it contains entries for `VSYSCALL_PAGE` - if emulation of legacy vsyscall page is enabled, `FIX_APIC_BASE` for local [apic](h
ttp://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) and etc... In a virtual memory fix-mapped area is placed in the modules area:
As you can see `level2_fixmap_pgt` is right after the `level2_kernel_pgt` which is kernel code+data+bss. Every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses` enum from the [arch/x86/include/asm/fixmap.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/fixmap.h). For example it contains entries for `VSYSCALL_PAGE` - if emulation of legacy vsyscall page is enabled, `FIX_APIC_BASE` for local [apic](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller), etc. In virtual memory fix-mapped area is placed in the modules area:
@ -39,13 +38,13 @@ Base virtual address and size of the `fix-mapped` area are presented by the two
Here `__end_of_permanent_fixed_addresses` is an element of the `fixed_addresses` enum and as I wrote above: Every fix-mapped address is represented by an integer index which is defined in the `fixed_addresses`. `PAGE_SHIFT` determines size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT`. In our case we need to get the size of the fix-mapped area, but not only of one page, that's why we are using `__end_of_permanent_fixed_addresses` for getting the size of the fix-mapped area. In my case it's a little more than `536` killobytes. In your case it might be a different number, because the size depends on amount of the fix-mapped addresses which are depends on your kernel's configuration.
The second `FIXADDR_START` macro just extracts from the last address of the fix-mapped area its size for getting base virtual address of the fix-mapped area. `FIXADDR_TOP` is rounded up address from the base address of the [vsyscall](https://lwn.net/Articles/446528/) space:
The second `FIXADDR_START` macro just substracts fix-mapped area size from the last address of the fix-mapped area to get its base virtual address. `FIXADDR_TOP` is a rounded up address from the base address of the [vsyscall](https://lwn.net/Articles/446528/) space:
The `fixed_addresses` enums are used as an index to get the virtual address using the `fix_to_virt` function. Implementation of this function is easy:
The `fixed_addresses` enums are used as an index to get the virtual address by the `fix_to_virt` function. Implementation of this function is easy:
```C
static __always_inline unsigned long fix_to_virt(const unsigned int idx)
@ -79,7 +78,7 @@ static inline unsigned long virt_to_fix(const unsigned long vaddr)
A PFN is simply an index within physical memory that is counted in page-sized units. PFN for a physical address could be trivially defined as (page_phys_addr >> PAGE_SHIFT);
`__virt_to_fix` clears the first 12 bits in the given address, subtracts it from the last address the of `fix-mapped` area (`FIXADDR_TOP`) and shifts right result on `PAGE_SHIFT` which is `12`. Let me explain how it works. As I already wrote we will clear the first 12 bits in the given address with `x & PAGE_MASK`. As we subtract this from the `FIXADDR_TOP`, we will get the last 12 bits of the `FIXADDR_TOP` which are present. We know that the first 12 bits of the virtual address represent the offset in the page frame. With the shiting it on `PAGE_SHIFT` we will get `Page frame number` which is just all bits in a virtual address besides the first 12 offset bits. `Fix-mapped` addresses are used in different [places](http://lxr.free-electrons.com/ident?i=fix_to_virt) in the linux kernel. `IDT` descriptor stored there, [Intel Trusted Execution Technology](http://en.wikipedia.org/wiki/Trusted_Execution_Technology) UUID stored in the `fix-mapped` area started from `FIX_TBOOT_BASE` index, [Xen](http://en.wikipedia.org/wiki/Xen) bootmap and many more... We already saw a little about `fix-mapped` addresses in the fifth [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-5.html) about linux kernel initialization. We used`fix-mapped` area in the early `ioremap` initialization. Let's look on it and try to understand what is it`ioremap`, how it is implemented in the kernel and how it is releated to the `fix-mapped` addresses.
`__virt_to_fix` clears the first 12 bits in the given address, subtracts it from the last address the of `fix-mapped` area (`FIXADDR_TOP`) and shifts the result right on `PAGE_SHIFT` which is `12`. Let me explain how it works. As I already wrote we will clear the first 12 bits in the given address with `x & PAGE_MASK`. As we subtract this from the `FIXADDR_TOP`, we will get the last 12 bits of the `FIXADDR_TOP` which are present. We know that the first 12 bits of the virtual address represent the offset in the page frame. With the shiting it on `PAGE_SHIFT` we will get `Page frame number` which is just all bits in a virtual address besides the first 12 offset bits. `Fix-mapped` addresses are used in different [places](http://lxr.free-electrons.com/ident?i=fix_to_virt) in the linux kernel. `IDT` descriptor stored there, [Intel Trusted Execution Technology](http://en.wikipedia.org/wiki/Trusted_Execution_Technology) UUID stored in the `fix-mapped` area started from `FIX_TBOOT_BASE` index, [Xen](http://en.wikipedia.org/wiki/Xen) bootmap and many more... We already saw a little about `fix-mapped` addresses in the fifth [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-5.html) about linux kernel initialization. We use `fix-mapped` area in the early `ioremap` initialization. Let's look on it and try to understand what is `ioremap`, how it is implemented in the kernel and how it is releated to the `fix-mapped` addresses.
@ -132,7 +131,7 @@ As we can see it takes three parameters:
* `n` - length of region;
* `name` - name of requester.
`request_region` allocates `I/O` port region. Very often `check_region` function called before the `request_region` to check that the given address range is available and `release_region` to release memory region. `request_region` returns pointer to the `resource` structure. `resource` structure presents abstraction for a tree-like subset of system resources. We already saw `resource` structure in the firth part about kernel [initialization](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-5.html) process and it looks as:
`request_region` allocates `I/O` port region. Very often `check_region` function is called before the `request_region` to check that the given address range is available and `release_region` to release memory region. `request_region` returns pointer to the `resource` structure. `resource` structure presents abstraction for a tree-like subset of system resources. We already saw `resource` structure in the firth part about kernel [initialization](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-5.html) process and it looks as:
```C
struct resource {
@ -144,7 +143,7 @@ struct resource {
};
```
and contains start and end addresses of the resource, name and etc... Every `resource` structure contains pointers to the `parent`, `slibling` and `child` resources. As it has parent and childs, it means that every subset of resuorces has root `resource` structure. For example, for `I/O` ports it is `ioport_resource` structure:
and contains start and end addresses of the resource, name, etc. Every `resource` structure contains pointers to the `parent`, `slibling` and `child` resources. As it has parent and childs, it means that every subset of resuorces has root `resource` structure. For example, for `I/O` ports it is `ioport_resource` structure:
As I wrote about `request_regions` is used for registering of I/O port region and this macro used in many [places](http://lxr.free-electrons.com/ident?i=request_region) in the kernel. For example let's look at [drivers/char/rtc.c](https://github.com/torvalds/linux/blob/master/char/rtc.c). This source code file provides [Real Time Clock](http://en.wikipedia.org/wiki/Real-time_clock) interface in the linux kernel. As every kernel module, `rtc` module contains `module_init` definition:
As I wrote about `request_regions` is used for registering of I/O port region and this macro is used in many [places](http://lxr.free-electrons.com/ident?i=request_region) in the kernel. For example let's look at [drivers/char/rtc.c](https://github.com/torvalds/linux/blob/master/char/rtc.c). This source code file provides [Real Time Clock](http://en.wikipedia.org/wiki/Real-time_clock) interface in the linux kernel. As every kernel module, `rtc` module contains `module_init` definition:
```C
module_init(rtc_init);
```
where `rtc_init` is `rtc` initialization function. This function defined in the same `rtc.c` source code file. In the `rtc_init` function we can see a couple calls of the `rtc_request_region` functions, which wrap `request_region` for example:
where `rtc_init` is `rtc` initialization function. This function is defined in the same `rtc.c` source code file. In the `rtc_init` function we can see a couple calls of the `rtc_request_region` functions, which wrap `request_region` for example:
```C
r = rtc_request_region(RTC_IO_EXTENT);
@ -203,7 +202,7 @@ So, we got it! Ok, it was ports. The second way is use of `I/O` memory. As I wro
* start of the memory region;
* size of the memory region;
I/O memory mapping API provides function for the checking, requesting and release of a memory region as this does I/O ports API. There are three functions for it:
I/O memory mapping API provides functions for checking, requesting and release of a memory region as I/O ports API. There are three functions for it:
* `request_mem_region`
* `release_mem_region`
@ -239,7 +238,7 @@ e0000000-feafffff : PCI Bus 0000:00
...
```
Part of these addresses is from the call of the `e820_reserve_resources` function. We can find call of this function in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) and the function itself defined in the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c). `e820_reserve_resources` goes through the [e820](http://en.wikipedia.org/wiki/E820) map and inserts memory regions to the root `iomem` resource structure. All `e820` memory regions which are will be inserted to the `iomem` resource will have following types:
Part of these addresses is from the call of the `e820_reserve_resources` function. We can find call of this function in the [arch/x86/kernel/setup.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.c) and the function itself is defined in the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c). `e820_reserve_resources` goes through the [e820](http://en.wikipedia.org/wiki/E820) map and inserts memory regions to the root `iomem` resource structure. All `e820` memory regions which will be inserted to the `iomem` resource have following types:
and we can see it in the `/proc/iomem` (read above).
and we can see them in the `/proc/iomem` (read above).
Now let's try to understand how `ioremap` works. We already know a little about `ioremap`, we saw it in the fifth [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-5.html) about linux kernel initialization. If you have read this part, you can remember the call of the `early_ioremap_init` function from the [arch/x86/mm/ioremap.c](https://github.com/torvalds/linux/blob/master/arch/x86/mm/ioremap.c). Initialization of the `ioremap` is split inn two parts: there is the early part which we can use before the normal `ioremap` is available and the normal `ioremap` which is available after `vmalloc` initialization and call of the `paging_init`. We do not know anything about `vmalloc` for now, so let's consider early initialization of the `ioremap`. First of all `early_ioremap_init` checks that `fixmap` is aligned on page middle directory boundary:
@ -295,7 +294,7 @@ static unsigned long prev_size[FIX_BTMAPS_SLOTS] __initdata;
static unsigned long slot_virt[FIX_BTMAPS_SLOTS] __initdata;
```
`slot_virt` contains virtual addresses of the `fix-mapped` areas, `prev_map` array contains addresses of the early ioremap areas. Note that I wrote above: `Actually there are 512 temporary boot-time mappings, used by early ioremap` and you can see that all arrays defined with the `__initdata` attribute which means that this memory will be released after kernel initialization process. After `early_ioremap_setup` finished to work, we're getting page middle directory where early ioremap beginning with the `early_ioremap_pmd` function which just gets the base address of the page global directory and calculates the page middle directory for the given address:
`slot_virt` contains virtual addresses of the `fix-mapped` areas, `prev_map` array contains addresses of the early ioremap areas. Note that I wrote above: `Actually there are 512 temporary boot-time mappings, used by early ioremap` and you can see that all arrays defined with the `__initdata` attribute which means that this memory will be released after kernel initialization process. After `early_ioremap_setup` finished its work, we're getting page middle directory where early ioremap begins with the `early_ioremap_pmd` function which just gets the base address of the page global directory and calculates the page middle directory for the given address:
```C
static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
@ -362,7 +361,7 @@ As early `ioremap` is setup, we can use it. It provides two functions:
* early_ioremap
* early_iounmap
for mapping/unmapping of IO physical address to virtual address. Both functions depends on `CONFIG_MMU` configuration option. [Memory management unit](http://en.wikipedia.org/wiki/Memory_management_unit) is a special block of memory management. Main purpose of this block is translation physical addresses to the virtual. Techinically memory management unit knows about high-level page table address (`pgd`) from the `cr3` control register. If `CONFIG_MMU` options is set to `n`, `early_ioremap` just returns the given physical address and `early_iounmap` does not nothing. In other way, if `CONFIG_MMU` option is set to `y`, `early_ioremap` calls `__early_ioremap` which takes three parameters:
for mapping/unmapping of IO physical address to virtual address. Both functions depends on `CONFIG_MMU` configuration option. [Memory management unit](http://en.wikipedia.org/wiki/Memory_management_unit) is a special block of memory management. Main purpose of this block is translation physical addresses to virtual adresses. Techinically memory management unit knows about high-level page table address (`pgd`) from the `cr3` control register. If `CONFIG_MMU` options is set to `n`, `early_ioremap` just returns the given physical address and `early_iounmap` does not nothing. In other way, if `CONFIG_MMU` option is set to `y`, `early_ioremap` calls `__early_ioremap` which takes three parameters:
* `phys_addr` - base physicall address of the `I/O` memory region to map on virtual addresses;
* `size` - size of the `I/O` memroy region;
@ -394,13 +393,13 @@ phys_addr &= PAGE_MASK;
size = PAGE_ALIGN(last_addr + 1) - phys_addr;
```
Here we are using `PAGE_MASK` for clearing all bits in the `phys_addr`besides first 12 bits. `PAGE_MASK` macro defined as:
Here we are using `PAGE_MASK` for clearing all bits in the `phys_addr`except the first 12 bits. `PAGE_MASK` macro is defined as:
```C
#define PAGE_MASK (~(PAGE_SIZE-1))
```
We know that size of a page is 4096 bytes or `1000000000000` in binary. `PAGE_SIZE - 1` will be `111111111111`, but with `~`, we will get `000000000000`, but as we use `~PAGE_MASK` we will get `111111111111` again. On the second line we do the same but clear first 12 bits and getting page-aligned size of the area on the third line. We getting aligned area and now we need to get the number of pages which are occupied by the new `ioremap` are and calculate the fix-mapped index from `fixed_addresses` in the next steps:
We know that size of a page is 4096 bytes or `1000000000000` in binary. `PAGE_SIZE - 1` will be `111111111111`, but with `~`, we will get `000000000000`, but as we use `~PAGE_MASK` we will get `111111111111` again. On the second line we do the same but clear the first 12 bits and getting page-aligned size of the area on the third line. We getting aligned area and now we need to get the number of pages which are occupied by the new `ioremap` area and calculate the fix-mapped index from `fixed_addresses` in the next steps:
```C
nrpages = size >> PAGE_SHIFT;
@ -445,7 +444,7 @@ flags, so we call `set_pte` function for setting page table entry which works in
__flush_tlb_one(addr);
```
This function defined in the [arch/x86/include/asm/tlbflush.h](https://github.com/torvalds/linux/blob/master) and calls `__flush_tlb_single` or `__flush_tlb` depends on value of the `cpu_has_invlpg`:
This function is defined in the [arch/x86/include/asm/tlbflush.h](https://github.com/torvalds/linux/blob/master) and calls `__flush_tlb_single` or `__flush_tlb` depends on value of the `cpu_has_invlpg`:
```C
static inline void __flush_tlb_one(unsigned long addr)
@ -457,13 +456,13 @@ static inline void __flush_tlb_one(unsigned long addr)
}
```
`__flush_tlb_one` function invalidates given address in the [TLB](http://en.wikipedia.org/wiki/Translation_lookaside_buffer). As you just saw we updated paging structure, but `TLB` not informed of changes, that's why we need to do it manually. There are two ways how to do it. First is update `cr3` control register and `__flush_tlb` function does this:
`__flush_tlb_one` function invalidates given address in the [TLB](http://en.wikipedia.org/wiki/Translation_lookaside_buffer). As you just saw we updated paging structure, but `TLB`is not informed of the changes, that's why we need to do it manually. There are two ways to do it. First is update `cr3` control register and `__flush_tlb` function does this:
```C
native_write_cr3(native_read_cr3());
```
The second method is to use `invlpg` instruction invalidates `TLB` entry. Let's look on `__flush_tlb_one` implementation. As you can see first of all it checks `cpu_has_invlpg` which defined as:
The second method is to use `invlpg` instruction to invalidates `TLB` entry. Let's look on `__flush_tlb_one` implementation. As you can see first of all it checks `cpu_has_invlpg` which defined as:
@ -482,7 +481,7 @@ static inline void __native_flush_tlb_single(unsigned long addr)
}
```
or call `__flush_tlb` which just updates `cr3` register as we saw it above. After this step execution of the `__early_set_fixmap` function is finsihed and we can back to the `__early_ioremap` implementation. As we set fixmap area for the given address, need to save the base virtual address of the I/O Re-mapped area in the `prev_map` with the `slot` index:
or call `__flush_tlb` which just updates `cr3` register as we saw it above. After this step execution of the `__early_set_fixmap` function is finsihed and we can back to the `__early_ioremap` implementation. As we have set fixmap area for the given address, we need to save the base virtual address of the I/O Re-mapped area in the `prev_map` with the `slot` index:
@ -496,7 +495,7 @@ The second function is - `early_iounmap` - unmaps an `I/O` memory region. This f
prev_map[slot] = NULL;
```
That's all about `fixmaps` and `ioremap`. Of course this part does not cover full features of the `ioremap`, it was only early ioremap, but there is also normal ioremap. But we need to know more things than now before it.
That's all about `fixmaps` and `ioremap`. Of course this part does not cover full features of the `ioremap`, it was only early ioremap, but there is also normal ioremap. But we need to know more things before it.
0xAX
8 years ago