From b9de3e899932f455c1cba6c45892be370b067d21 Mon Sep 17 00:00:00 2001 From: 0xAX <0xAX@users.noreply.github.com> Date: Tue, 8 Nov 2016 00:37:12 +0600 Subject: [PATCH] Update linux-mm-2.md --- MM/linux-mm-2.md | 32 +++++++++++++++++++++++++------- 1 file changed, 25 insertions(+), 7 deletions(-) diff --git a/MM/linux-mm-2.md b/MM/linux-mm-2.md index 341f7d3..f43e958 100644 --- a/MM/linux-mm-2.md +++ b/MM/linux-mm-2.md @@ -33,10 +33,12 @@ Base virtual address and size of the `fix-mapped` area are presented by the two ```C #define FIXADDR_SIZE (__end_of_permanent_fixed_addresses << PAGE_SHIFT) -#define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE) +#define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE) ``` -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 the 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` kilobytes. 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. +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 the size of a page. For example size of the one page we can get with the `1 << PAGE_SHIFT` expression. + +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. The `__end_of_permanent_fixed_addresses` is the last index of the `fixed_addresses` enum or in other words the `__end_of_permanent_fixed_addresses` contains amount of pages in a fixed-mapped area. So if multiply value of the `__end_of_permanent_fixed_addresses` on a page size value we will get size of fix-mapped area. In my case it's a little more than `536` kilobytes. 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 subtracts the 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: @@ -60,7 +62,19 @@ first of all it checks that the index given for the `fixed_addresses` enum is no #define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT)) ``` -Here we shift left the given `fix-mapped` address index on the `PAGE_SHIFT` which determines size of a page as I wrote above and subtract it from the `FIXADDR_TOP` which is the highest address of the `fix-mapped` area. There is an inverse function for getting `fix-mapped` address from a virtual address: +Here we shift left the given index of a `fix-mapped` area on the `PAGE_SHIFT` which determines size of a page as I wrote above and subtract it from the `FIXADDR_TOP` which is the highest address of the `fix-mapped` area: + +``` ++-----------------+ +| PAGE 1 | FIXADDR_TOP (virt address) +| PAGE 2 | +| PAGE 3 | +| PAGE 4 (idx) | x - 4 +| PAGE 5 | ++-----------------+ +``` + +There is an inverse function for getting an index of a fix-mapped area corresponding to the given virtual address: ```C static inline unsigned long virt_to_fix(const unsigned long vaddr) @@ -70,15 +84,19 @@ static inline unsigned long virt_to_fix(const unsigned long vaddr) } ``` -`virt_to_fix` takes a virtual address, checks that this address is between `FIXADDR_START` and `FIXADDR_TOP` and calls the `__virt_to_fix` macro which implemented as: +The `virt_to_fix` takes a virtual address, checks that this address is between `FIXADDR_START` and `FIXADDR_TOP` and calls the `__virt_to_fix` macro which implemented as: ```C #define __virt_to_fix(x) ((FIXADDR_TOP - ((x)&PAGE_MASK)) >> PAGE_SHIFT) ``` -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); +As we may see, the `__virt_to_fix` macro clears the first `12` bits in the given virtual 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 in previous example (in `__fix_to_virt` macro), we start from the top of the fix-mapped area. We also go back to bottom from the top to search an index of a fix-mapped area corresponding to the given virtual address. As you may see, forst of all we will clear the first `12` bits in the given virtual address with `x & PAGE_MASK` expression. This allows us to get base address of page. We need to do this for case when the given virtual address points somewhere in a beginning/middle or end of a page, but not to the base address of it. At the next step subtract this from the `FIXADDR_TOP` and this gives us virtual address of a correspinding page in a fix-mapped area. In the end we just divide value of this address on `PAGE_SHIFT`. This gives us index of a fix-mapped area corresponding to the given virtual address. It may looks hard, but if you will go through this step by step, you will be sure that the `__virt_to_fix` macro is pretty easy. + +That's all. For this moment we know a little about `fix-mapped` addresses, but this is enough to go next. -`__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 shifting 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 of the linux kernel initialization. We use `fix-mapped` area in the early `ioremap` initialization. Let's look at it more closely and try to understand what `ioremap` is, how it is implemented in the kernel and how it is related to the `fix-mapped` addresses. +`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 of the linux kernel initialization. We use `fix-mapped` area in the early `ioremap` initialization. Let's look at it more closely and try to understand what `ioremap` is, how it is implemented in the kernel and how it is related to the `fix-mapped` addresses. ioremap -------------------------------------------------------------------------------- @@ -518,4 +536,4 @@ Links * [Memory management unit](http://en.wikipedia.org/wiki/Memory_management_unit) * [TLB](http://en.wikipedia.org/wiki/Translation_lookaside_buffer) * [Paging](http://0xax.gitbooks.io/linux-insides/content/Theory/Paging.html) -* [Linux kernel memory management Part 1.](http://0xax.gitbooks.io/linux-insides/content/MM/linux-mm-1.html) +* [Linux kernel memory management Part 1.](http://0xax.gitbooks.io/linux-insides/content/mm/linux-mm-1.html)