linux-insides/mm/linux-mm-2.md

Linux kernel memory management Part 2.

Fix-Mapped Addresses and ioremap

Fix-Mapped addresses are a set of special compile-time addresses whose corresponding physical address do not have to be a linear address minus __START_KERNEL_map. Each fix-mapped address maps one page frame and the kernel uses them as pointers that never change their address. That is the main point of these addresses. As the comment says: to have a constant address at compile time, but to set the physical address only in the boot process. You can remember that in the earliest part, we already set the level2_fixmap_pgt:

NEXT_PAGE(level2_fixmap_pgt)
	.fill	506,8,0
	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE
	.fill	5,8,0

NEXT_PAGE(level1_fixmap_pgt)
	.fill	512,8,0

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. For example it contains entries for VSYSCALL_PAGE - if emulation of legacy vsyscall page is enabled, FIX_APIC_BASE for local apic, etc. In virtual memory fix-mapped area is placed in the modules area:

       +-----------+-----------------+---------------+------------------+
       |           |                 |               |                  |
	   |kernel text|      kernel     |               |    vsyscalls     |
	   | mapping   |       text      |    Modules    |    fix-mapped    |
       |from phys 0|       data      |               |    addresses     |
       |           |                 |               |                  |
       +-----------+-----------------+---------------+------------------+
__START_KERNEL_map   __START_KERNEL    MODULES_VADDR            0xffffffffffffffff

Base virtual address and size of the fix-mapped area are presented by the two following macro:

#define FIXADDR_SIZE	(__end_of_permanent_fixed_addresses << PAGE_SHIFT)
#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 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.

The second FIXADDR_START macro just subtracts 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 space:

#define FIXADDR_TOP     (round_up(VSYSCALL_ADDR + PAGE_SIZE, 1<<PMD_SHIFT) - PAGE_SIZE)

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:

static __always_inline unsigned long fix_to_virt(const unsigned int idx)
{
        BUILD_BUG_ON(idx >= __end_of_fixed_addresses);
        return __fix_to_virt(idx);
}

first of all it checks that the index given for the fixed_addresses enum is not greater or equal than __end_of_fixed_addresses with the BUILD_BUG_ON macro and then returns the result of the __fix_to_virt macro:

#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:

static inline unsigned long virt_to_fix(const unsigned long vaddr)
{
        BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
        return __virt_to_fix(vaddr);
}

virt_to_fix takes virtual address, checks that this address is between FIXADDR_START and FIXADDR_TOP and calls __virt_to_fix macro which implemented as:

#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);

__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 in the linux kernel. IDT descriptor stored there, Intel Trusted Execution Technology UUID stored in the fix-mapped area started from FIX_TBOOT_BASE index, Xen bootmap and many more... We already saw a little about fix-mapped addresses in the fifth part 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 related to the fix-mapped addresses.

ioremap

Linux kernel provides many different primitives to manage memory. For this moment we will touch I/O memory. Every device is controlled by reading/writing from/to its registers. For example a driver can turn off/on a device by writing to its registers or get the state of a device by reading from its registers. Besides registers, many devices have buffers where a driver can write something or read from there. As we know for this moment there are two ways to access device's registers and data buffers:

• through the I/O ports;
• mapping of the all registers to the memory address space;

In the first case every control register of a device has a number of input and output port. And driver of a device can read from a port and write to it with two in and out instructions which we already saw. If you want to know about currently registered port regions, you can know they by accessing of /proc/ioports:

$ cat /proc/ioports
0000-0cf7 : PCI Bus 0000:00
  0000-001f : dma1
  0020-0021 : pic1
  0040-0043 : timer0
  0050-0053 : timer1
  0060-0060 : keyboard
  0064-0064 : keyboard
  0070-0077 : rtc0
  0080-008f : dma page reg
  00a0-00a1 : pic2
  00c0-00df : dma2
  00f0-00ff : fpu
    00f0-00f0 : PNP0C04:00
  03c0-03df : vesafb
  03f8-03ff : serial
  04d0-04d1 : pnp 00:06
  0800-087f : pnp 00:01
  0a00-0a0f : pnp 00:04
  0a20-0a2f : pnp 00:04
  0a30-0a3f : pnp 00:04
0cf8-0cff : PCI conf1
0d00-ffff : PCI Bus 0000:00
...
...
...

/proc/ioporst provides information about what driver used address of a I/O ports region. All of these memory regions, for example 0000-0cf7, were claimed with the request_region function from the include/linux/ioport.h. Actually request_region is a macro which defied as:

#define request_region(start,n,name)   __request_region(&ioport_resource, (start), (n), (name), 0)

As we can see it takes three parameters:

• start - begin of region;
• n - length of region;
• name - name of requester.

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 process and it looks as:

struct resource {
        resource_size_t start;
        resource_size_t end;
        const char *name;
        unsigned long flags;
        struct resource *parent, *sibling, *child;
};

and contains start and end addresses of the resource, name, etc. Every resource structure contains pointers to the parent, sibling and child resources. As it has parent and childs, it means that every subset of resources has root resource structure. For example, for I/O ports it is ioport_resource structure:

struct resource ioport_resource = {
         .name   = "PCI IO",
         .start  = 0,
         .end    = IO_SPACE_LIMIT,
        .flags  = IORESOURCE_IO,
};
EXPORT_SYMBOL(ioport_resource);

Or for iomem, it is iomem_resource structure:

struct resource iomem_resource = {
        .name   = "PCI mem",
        .start  = 0,
        .end    = -1,
        .flags  = IORESOURCE_MEM,
};

As I wrote about request_regions is used for registering of I/O port region and this macro is used in many places in the kernel. For example let's look at drivers/char/rtc.c. This source code file provides Real Time Clock interface in the linux kernel. As every kernel module, rtc module contains module_init definition:

module_init(rtc_init);

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:

r = rtc_request_region(RTC_IO_EXTENT);

where rtc_request_region calls:

r = request_region(RTC_PORT(0), size, "rtc");

Here RTC_IO_EXTENT is a size of memory region and it is 0x8, "rtc" is a name of region and RTC_PORT is:

#define RTC_PORT(x)     (0x70 + (x))

So with the request_region(RTC_PORT(0), size, "rtc") we register memory region, started at 0x70 and with size 0x8. Let's look on the /proc/ioports:

~$ sudo cat /proc/ioports | grep rtc
0070-0077 : rtc0

So, we got it! Ok, it was ports. The second way is use of I/O memory. As I wrote above this way is mapping of control registers and memory of a device to the memory address space. I/O memory is a set of contiguous addresses which are provided by a device to CPU through a bus. All memory-mapped I/O addresses are not used by the kernel directly. There is a special ioremap function which allows us to covert the physical address on a bus to the kernel virtual address or in another words ioremap maps I/O physical memory region to access it from the kernel. The ioremap function takes two parameters:

• start of the memory region;
• size of the memory region;

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
• check_mem_region

~$ sudo cat /proc/iomem
...
...
...
be826000-be82cfff : ACPI Non-volatile Storage
be82d000-bf744fff : System RAM
bf745000-bfff4fff : reserved
bfff5000-dc041fff : System RAM
dc042000-dc0d2fff : reserved
dc0d3000-dc138fff : System RAM
dc139000-dc27dfff : ACPI Non-volatile Storage
dc27e000-deffefff : reserved
defff000-deffffff : System RAM
df000000-dfffffff : RAM buffer
e0000000-feafffff : PCI Bus 0000:00
  e0000000-efffffff : PCI Bus 0000:01
    e0000000-efffffff : 0000:01:00.0
  f7c00000-f7cfffff : PCI Bus 0000:06
    f7c00000-f7c0ffff : 0000:06:00.0
    f7c10000-f7c101ff : 0000:06:00.0
      f7c10000-f7c101ff : ahci
  f7d00000-f7dfffff : PCI Bus 0000:03
    f7d00000-f7d3ffff : 0000:03:00.0
      f7d00000-f7d3ffff : alx
...
...
...

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 and the function itself is defined in the arch/x86/kernel/e820.c. e820_reserve_resources goes through the 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:

static inline const char *e820_type_to_string(int e820_type)
{
	switch (e820_type) {
	case E820_RESERVED_KERN:
	case E820_RAM:	return "System RAM";
	case E820_ACPI:	return "ACPI Tables";
	case E820_NVS:	return "ACPI Non-volatile Storage";
	case E820_UNUSABLE:	return "Unusable memory";
	default:	return "reserved";
	}
}

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 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. 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:

BUILD_BUG_ON((fix_to_virt(0) + PAGE_SIZE) & ((1 << PMD_SHIFT) - 1));

more about BUILD_BUG_ON you can read in the first part about Linux Kernel initialization. So BUILD_BUG_ON macro raises compilation error if the given expression is true. In the next step after this check, we can see call of the early_ioremap_setup function from the mm/early_ioremap.c. This function presents generic initialization of the ioremap. early_ioremap_setup function fills the slot_virt array with the virtual addresses of the early fixmaps. All early fixmaps are after __end_of_permanent_fixed_addresses in memory. They are stats from the FIX_BITMAP_BEGIN (top) and ends with FIX_BITMAP_END (down). Actually there are 512 temporary boot-time mappings, used by early ioremap:

#define NR_FIX_BTMAPS		64
#define FIX_BTMAPS_SLOTS	8
#define TOTAL_FIX_BTMAPS	(NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)

and early_ioremap_setup:

void __init early_ioremap_setup(void)
{
        int i;

        for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
                if (WARN_ON(prev_map[i]))
                        break;

        for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
                slot_virt[i] = __fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);
}

the slot_virt and other arrays are defined in the same source code file:

static void __iomem *prev_map[FIX_BTMAPS_SLOTS] __initdata;
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 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:

static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
{
	pgd_t *base = __va(read_cr3());
	pgd_t *pgd = &base[pgd_index(addr)];
	pud_t *pud = pud_offset(pgd, addr);
	pmd_t *pmd = pmd_offset(pud, addr);
	return pmd;
}

After this we fills bm_pte (early ioremap page table entries) with zeros and call the pmd_populate_kernel function:

pmd = early_ioremap_pmd(fix_to_virt(FIX_BTMAP_BEGIN));
memset(bm_pte, 0, sizeof(bm_pte));
pmd_populate_kernel(&init_mm, pmd, bm_pte);

pmd_populate_kernel takes three parameters:

• init_mm - memory descriptor of the init process (you can read about it in the previous part);
• pmd - page middle directory of the beginning of the ioremap fixmaps;
• bm_pte - early ioremap page table entries array which defined as:

static pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)] __page_aligned_bss;

The pmd_popularte_kernel function defined in the arch/x86/include/asm/pgalloc.h and populates given page middle directory (pmd) with the given page table entries (bm_pte):

static inline void pmd_populate_kernel(struct mm_struct *mm,
                                       pmd_t *pmd, pte_t *pte)
{
        paravirt_alloc_pte(mm, __pa(pte) >> PAGE_SHIFT);
        set_pmd(pmd, __pmd(__pa(pte) | _PAGE_TABLE));
}

where set_pmd is:

#define set_pmd(pmdp, pmd)              native_set_pmd(pmdp, pmd)

and native_set_pmd is:

static inline void native_set_pmd(pmd_t *pmdp, pmd_t pmd)
{
        *pmdp = pmd;
}

That's all. Early ioremap is ready to use. There are a couple of checks in the early_ioremap_init function, but they are not so important, anyway initialization of the ioremap is finished.

Use of early ioremap

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 is a special block of memory management. Main purpose of this block is translation physical addresses to virtual addresses. Technically 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 physical address of the I/O memory region to map on virtual addresses;
• size - size of the I/O memory region;
• prot - page table entry bits.

First of all in the __early_ioremap, we goes through the all early ioremap fixmap slots and check first free are in the prev_map array and remember it's number in the slot variable and set up size as we found it:

slot = -1;
for (i = 0; i < FIX_BTMAPS_SLOTS; i++) {
	if (!prev_map[i]) {
		slot = i;
		break;
	}
}
...
...
...
prev_size[slot] = size;
last_addr = phys_addr + size - 1;

In the next spte we can see the following code:

offset = phys_addr & ~PAGE_MASK;
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 except the first 12 bits. PAGE_MASK macro is defined as:

#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 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:

nrpages = size >> PAGE_SHIFT;
idx = FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*slot;

Now we can fill fix-mapped area with the given physical addresses. Every iteration in the loop, we call __early_set_fixmap function from the arch/x86/mm/ioremap.c, increase given physical address on page size which is 4096 bytes and update addresses index and number of pages:

while (nrpages > 0) {
	__early_set_fixmap(idx, phys_addr, prot);
	phys_addr += PAGE_SIZE;
	--idx;
    --nrpages;
}

The __early_set_fixmap function gets the page table entry (stored in the bm_pte, see above) for the given physical address with:

pte = early_ioremap_pte(addr);

In the next step of the early_ioremap_pte we check the given page flags with the pgprot_val macro and calls set_pte or pte_clear depends on it:

if (pgprot_val(flags))
		set_pte(pte, pfn_pte(phys >> PAGE_SHIFT, flags));
	else
		pte_clear(&init_mm, addr, pte);

As you can see above, we passed FIXMAP_PAGE_IO as flags to the __early_ioremap. FIXMPA_PAGE_IO expands to the:

(__PAGE_KERNEL_EXEC | _PAGE_NX)

flags, so we call set_pte function for setting page table entry which works in the same manner as set_pmd but for PTEs (read above about it). As we set all PTEs in the loop, we can see the call of the __flush_tlb_one function:

__flush_tlb_one(addr);

This function is defined in the arch/x86/include/asm/tlbflush.h and calls __flush_tlb_single or __flush_tlb depends on value of the cpu_has_invlpg:

static inline void __flush_tlb_one(unsigned long addr)
{
        if (cpu_has_invlpg)
                __flush_tlb_single(addr);
        else
                __flush_tlb();
}

__flush_tlb_one function invalidates given address in the TLB. 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:

native_write_cr3(native_read_cr3());

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:

#if defined(CONFIG_X86_INVLPG) || defined(CONFIG_X86_64)
# define cpu_has_invlpg         1
#else
# define cpu_has_invlpg         (boot_cpu_data.x86 > 3)
#endif

If a CPU support invlpg instruction, we call the __flush_tlb_single macro which expands to the call of the __native_flush_tlb_single:

static inline void __native_flush_tlb_single(unsigned long addr)
{
        asm volatile("invlpg (%0)" ::"r" (addr) : "memory");
}

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 finished 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:

prev_map[slot] = (void __iomem *)(offset + slot_virt[slot]);

and return it.

The second function is - early_iounmap - unmaps an I/O memory region. This function takes two parameters: base address and size of a I/O region and generally looks very similar on early_ioremap. It also goes through fixmap slots and looks for slot with the given address. After this it gets the index of the fixmap slot and calls __late_clear_fixmap or __early_set_fixmap depends on after_paging_init value. It calls __early_set_fixmap with on difference then it does early_ioremap: it passes zero as physical address. And in the end it sets address of the I/O memory region to NULL:

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 before it.

So, this is the end!

Conclusion

This is the end of the second part about linux kernel memory management. If you have questions or suggestions, ping me on twitter 0xAX, drop me an email or just create an issue.

Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me a PR to linux-insides.

Links

• apic
• vsyscall
• Intel Trusted Execution Technology
• Xen
• Real Time Clock
• e820
• Memory management unit
• TLB
• Paging
• Linux kernel memory management Part 1.