403 lines
17 KiB
Text
403 lines
17 KiB
Text
Cache and TLB Flushing
|
|
Under Linux
|
|
|
|
David S. Miller <davem@redhat.com>
|
|
|
|
This document describes the cache/tlb flushing interfaces called
|
|
by the Linux VM subsystem. It enumerates over each interface,
|
|
describes its intended purpose, and what side effect is expected
|
|
after the interface is invoked.
|
|
|
|
The side effects described below are stated for a uniprocessor
|
|
implementation, and what is to happen on that single processor. The
|
|
SMP cases are a simple extension, in that you just extend the
|
|
definition such that the side effect for a particular interface occurs
|
|
on all processors in the system. Don't let this scare you into
|
|
thinking SMP cache/tlb flushing must be so inefficient, this is in
|
|
fact an area where many optimizations are possible. For example,
|
|
if it can be proven that a user address space has never executed
|
|
on a cpu (see mm_cpumask()), one need not perform a flush
|
|
for this address space on that cpu.
|
|
|
|
First, the TLB flushing interfaces, since they are the simplest. The
|
|
"TLB" is abstracted under Linux as something the cpu uses to cache
|
|
virtual-->physical address translations obtained from the software
|
|
page tables. Meaning that if the software page tables change, it is
|
|
possible for stale translations to exist in this "TLB" cache.
|
|
Therefore when software page table changes occur, the kernel will
|
|
invoke one of the following flush methods _after_ the page table
|
|
changes occur:
|
|
|
|
1) void flush_tlb_all(void)
|
|
|
|
The most severe flush of all. After this interface runs,
|
|
any previous page table modification whatsoever will be
|
|
visible to the cpu.
|
|
|
|
This is usually invoked when the kernel page tables are
|
|
changed, since such translations are "global" in nature.
|
|
|
|
2) void flush_tlb_mm(struct mm_struct *mm)
|
|
|
|
This interface flushes an entire user address space from
|
|
the TLB. After running, this interface must make sure that
|
|
any previous page table modifications for the address space
|
|
'mm' will be visible to the cpu. That is, after running,
|
|
there will be no entries in the TLB for 'mm'.
|
|
|
|
This interface is used to handle whole address space
|
|
page table operations such as what happens during
|
|
fork, and exec.
|
|
|
|
3) void flush_tlb_range(struct vm_area_struct *vma,
|
|
unsigned long start, unsigned long end)
|
|
|
|
Here we are flushing a specific range of (user) virtual
|
|
address translations from the TLB. After running, this
|
|
interface must make sure that any previous page table
|
|
modifications for the address space 'vma->vm_mm' in the range
|
|
'start' to 'end-1' will be visible to the cpu. That is, after
|
|
running, there will be no entries in the TLB for 'mm' for
|
|
virtual addresses in the range 'start' to 'end-1'.
|
|
|
|
The "vma" is the backing store being used for the region.
|
|
Primarily, this is used for munmap() type operations.
|
|
|
|
The interface is provided in hopes that the port can find
|
|
a suitably efficient method for removing multiple page
|
|
sized translations from the TLB, instead of having the kernel
|
|
call flush_tlb_page (see below) for each entry which may be
|
|
modified.
|
|
|
|
4) void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
|
|
|
|
This time we need to remove the PAGE_SIZE sized translation
|
|
from the TLB. The 'vma' is the backing structure used by
|
|
Linux to keep track of mmap'd regions for a process, the
|
|
address space is available via vma->vm_mm. Also, one may
|
|
test (vma->vm_flags & VM_EXEC) to see if this region is
|
|
executable (and thus could be in the 'instruction TLB' in
|
|
split-tlb type setups).
|
|
|
|
After running, this interface must make sure that any previous
|
|
page table modification for address space 'vma->vm_mm' for
|
|
user virtual address 'addr' will be visible to the cpu. That
|
|
is, after running, there will be no entries in the TLB for
|
|
'vma->vm_mm' for virtual address 'addr'.
|
|
|
|
This is used primarily during fault processing.
|
|
|
|
5) void update_mmu_cache(struct vm_area_struct *vma,
|
|
unsigned long address, pte_t *ptep)
|
|
|
|
At the end of every page fault, this routine is invoked to
|
|
tell the architecture specific code that a translation
|
|
now exists at virtual address "address" for address space
|
|
"vma->vm_mm", in the software page tables.
|
|
|
|
A port may use this information in any way it so chooses.
|
|
For example, it could use this event to pre-load TLB
|
|
translations for software managed TLB configurations.
|
|
The sparc64 port currently does this.
|
|
|
|
6) void tlb_migrate_finish(struct mm_struct *mm)
|
|
|
|
This interface is called at the end of an explicit
|
|
process migration. This interface provides a hook
|
|
to allow a platform to update TLB or context-specific
|
|
information for the address space.
|
|
|
|
The ia64 sn2 platform is one example of a platform
|
|
that uses this interface.
|
|
|
|
Next, we have the cache flushing interfaces. In general, when Linux
|
|
is changing an existing virtual-->physical mapping to a new value,
|
|
the sequence will be in one of the following forms:
|
|
|
|
1) flush_cache_mm(mm);
|
|
change_all_page_tables_of(mm);
|
|
flush_tlb_mm(mm);
|
|
|
|
2) flush_cache_range(vma, start, end);
|
|
change_range_of_page_tables(mm, start, end);
|
|
flush_tlb_range(vma, start, end);
|
|
|
|
3) flush_cache_page(vma, addr, pfn);
|
|
set_pte(pte_pointer, new_pte_val);
|
|
flush_tlb_page(vma, addr);
|
|
|
|
The cache level flush will always be first, because this allows
|
|
us to properly handle systems whose caches are strict and require
|
|
a virtual-->physical translation to exist for a virtual address
|
|
when that virtual address is flushed from the cache. The HyperSparc
|
|
cpu is one such cpu with this attribute.
|
|
|
|
The cache flushing routines below need only deal with cache flushing
|
|
to the extent that it is necessary for a particular cpu. Mostly,
|
|
these routines must be implemented for cpus which have virtually
|
|
indexed caches which must be flushed when virtual-->physical
|
|
translations are changed or removed. So, for example, the physically
|
|
indexed physically tagged caches of IA32 processors have no need to
|
|
implement these interfaces since the caches are fully synchronized
|
|
and have no dependency on translation information.
|
|
|
|
Here are the routines, one by one:
|
|
|
|
1) void flush_cache_mm(struct mm_struct *mm)
|
|
|
|
This interface flushes an entire user address space from
|
|
the caches. That is, after running, there will be no cache
|
|
lines associated with 'mm'.
|
|
|
|
This interface is used to handle whole address space
|
|
page table operations such as what happens during exit and exec.
|
|
|
|
2) void flush_cache_dup_mm(struct mm_struct *mm)
|
|
|
|
This interface flushes an entire user address space from
|
|
the caches. That is, after running, there will be no cache
|
|
lines associated with 'mm'.
|
|
|
|
This interface is used to handle whole address space
|
|
page table operations such as what happens during fork.
|
|
|
|
This option is separate from flush_cache_mm to allow some
|
|
optimizations for VIPT caches.
|
|
|
|
3) void flush_cache_range(struct vm_area_struct *vma,
|
|
unsigned long start, unsigned long end)
|
|
|
|
Here we are flushing a specific range of (user) virtual
|
|
addresses from the cache. After running, there will be no
|
|
entries in the cache for 'vma->vm_mm' for virtual addresses in
|
|
the range 'start' to 'end-1'.
|
|
|
|
The "vma" is the backing store being used for the region.
|
|
Primarily, this is used for munmap() type operations.
|
|
|
|
The interface is provided in hopes that the port can find
|
|
a suitably efficient method for removing multiple page
|
|
sized regions from the cache, instead of having the kernel
|
|
call flush_cache_page (see below) for each entry which may be
|
|
modified.
|
|
|
|
4) void flush_cache_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)
|
|
|
|
This time we need to remove a PAGE_SIZE sized range
|
|
from the cache. The 'vma' is the backing structure used by
|
|
Linux to keep track of mmap'd regions for a process, the
|
|
address space is available via vma->vm_mm. Also, one may
|
|
test (vma->vm_flags & VM_EXEC) to see if this region is
|
|
executable (and thus could be in the 'instruction cache' in
|
|
"Harvard" type cache layouts).
|
|
|
|
The 'pfn' indicates the physical page frame (shift this value
|
|
left by PAGE_SHIFT to get the physical address) that 'addr'
|
|
translates to. It is this mapping which should be removed from
|
|
the cache.
|
|
|
|
After running, there will be no entries in the cache for
|
|
'vma->vm_mm' for virtual address 'addr' which translates
|
|
to 'pfn'.
|
|
|
|
This is used primarily during fault processing.
|
|
|
|
5) void flush_cache_kmaps(void)
|
|
|
|
This routine need only be implemented if the platform utilizes
|
|
highmem. It will be called right before all of the kmaps
|
|
are invalidated.
|
|
|
|
After running, there will be no entries in the cache for
|
|
the kernel virtual address range PKMAP_ADDR(0) to
|
|
PKMAP_ADDR(LAST_PKMAP).
|
|
|
|
This routing should be implemented in asm/highmem.h
|
|
|
|
6) void flush_cache_vmap(unsigned long start, unsigned long end)
|
|
void flush_cache_vunmap(unsigned long start, unsigned long end)
|
|
|
|
Here in these two interfaces we are flushing a specific range
|
|
of (kernel) virtual addresses from the cache. After running,
|
|
there will be no entries in the cache for the kernel address
|
|
space for virtual addresses in the range 'start' to 'end-1'.
|
|
|
|
The first of these two routines is invoked after map_vm_area()
|
|
has installed the page table entries. The second is invoked
|
|
before unmap_kernel_range() deletes the page table entries.
|
|
|
|
There exists another whole class of cpu cache issues which currently
|
|
require a whole different set of interfaces to handle properly.
|
|
The biggest problem is that of virtual aliasing in the data cache
|
|
of a processor.
|
|
|
|
Is your port susceptible to virtual aliasing in its D-cache?
|
|
Well, if your D-cache is virtually indexed, is larger in size than
|
|
PAGE_SIZE, and does not prevent multiple cache lines for the same
|
|
physical address from existing at once, you have this problem.
|
|
|
|
If your D-cache has this problem, first define asm/shmparam.h SHMLBA
|
|
properly, it should essentially be the size of your virtually
|
|
addressed D-cache (or if the size is variable, the largest possible
|
|
size). This setting will force the SYSv IPC layer to only allow user
|
|
processes to mmap shared memory at address which are a multiple of
|
|
this value.
|
|
|
|
NOTE: This does not fix shared mmaps, check out the sparc64 port for
|
|
one way to solve this (in particular SPARC_FLAG_MMAPSHARED).
|
|
|
|
Next, you have to solve the D-cache aliasing issue for all
|
|
other cases. Please keep in mind that fact that, for a given page
|
|
mapped into some user address space, there is always at least one more
|
|
mapping, that of the kernel in its linear mapping starting at
|
|
PAGE_OFFSET. So immediately, once the first user maps a given
|
|
physical page into its address space, by implication the D-cache
|
|
aliasing problem has the potential to exist since the kernel already
|
|
maps this page at its virtual address.
|
|
|
|
void copy_user_page(void *to, void *from, unsigned long addr, struct page *page)
|
|
void clear_user_page(void *to, unsigned long addr, struct page *page)
|
|
|
|
These two routines store data in user anonymous or COW
|
|
pages. It allows a port to efficiently avoid D-cache alias
|
|
issues between userspace and the kernel.
|
|
|
|
For example, a port may temporarily map 'from' and 'to' to
|
|
kernel virtual addresses during the copy. The virtual address
|
|
for these two pages is chosen in such a way that the kernel
|
|
load/store instructions happen to virtual addresses which are
|
|
of the same "color" as the user mapping of the page. Sparc64
|
|
for example, uses this technique.
|
|
|
|
The 'addr' parameter tells the virtual address where the
|
|
user will ultimately have this page mapped, and the 'page'
|
|
parameter gives a pointer to the struct page of the target.
|
|
|
|
If D-cache aliasing is not an issue, these two routines may
|
|
simply call memcpy/memset directly and do nothing more.
|
|
|
|
void flush_dcache_page(struct page *page)
|
|
|
|
Any time the kernel writes to a page cache page, _OR_
|
|
the kernel is about to read from a page cache page and
|
|
user space shared/writable mappings of this page potentially
|
|
exist, this routine is called.
|
|
|
|
NOTE: This routine need only be called for page cache pages
|
|
which can potentially ever be mapped into the address
|
|
space of a user process. So for example, VFS layer code
|
|
handling vfs symlinks in the page cache need not call
|
|
this interface at all.
|
|
|
|
The phrase "kernel writes to a page cache page" means,
|
|
specifically, that the kernel executes store instructions
|
|
that dirty data in that page at the page->virtual mapping
|
|
of that page. It is important to flush here to handle
|
|
D-cache aliasing, to make sure these kernel stores are
|
|
visible to user space mappings of that page.
|
|
|
|
The corollary case is just as important, if there are users
|
|
which have shared+writable mappings of this file, we must make
|
|
sure that kernel reads of these pages will see the most recent
|
|
stores done by the user.
|
|
|
|
If D-cache aliasing is not an issue, this routine may
|
|
simply be defined as a nop on that architecture.
|
|
|
|
There is a bit set aside in page->flags (PG_arch_1) as
|
|
"architecture private". The kernel guarantees that,
|
|
for pagecache pages, it will clear this bit when such
|
|
a page first enters the pagecache.
|
|
|
|
This allows these interfaces to be implemented much more
|
|
efficiently. It allows one to "defer" (perhaps indefinitely)
|
|
the actual flush if there are currently no user processes
|
|
mapping this page. See sparc64's flush_dcache_page and
|
|
update_mmu_cache implementations for an example of how to go
|
|
about doing this.
|
|
|
|
The idea is, first at flush_dcache_page() time, if
|
|
page->mapping->i_mmap is an empty tree and ->i_mmap_nonlinear
|
|
an empty list, just mark the architecture private page flag bit.
|
|
Later, in update_mmu_cache(), a check is made of this flag bit,
|
|
and if set the flush is done and the flag bit is cleared.
|
|
|
|
IMPORTANT NOTE: It is often important, if you defer the flush,
|
|
that the actual flush occurs on the same CPU
|
|
as did the cpu stores into the page to make it
|
|
dirty. Again, see sparc64 for examples of how
|
|
to deal with this.
|
|
|
|
void copy_to_user_page(struct vm_area_struct *vma, struct page *page,
|
|
unsigned long user_vaddr,
|
|
void *dst, void *src, int len)
|
|
void copy_from_user_page(struct vm_area_struct *vma, struct page *page,
|
|
unsigned long user_vaddr,
|
|
void *dst, void *src, int len)
|
|
When the kernel needs to copy arbitrary data in and out
|
|
of arbitrary user pages (f.e. for ptrace()) it will use
|
|
these two routines.
|
|
|
|
Any necessary cache flushing or other coherency operations
|
|
that need to occur should happen here. If the processor's
|
|
instruction cache does not snoop cpu stores, it is very
|
|
likely that you will need to flush the instruction cache
|
|
for copy_to_user_page().
|
|
|
|
void flush_anon_page(struct vm_area_struct *vma, struct page *page,
|
|
unsigned long vmaddr)
|
|
When the kernel needs to access the contents of an anonymous
|
|
page, it calls this function (currently only
|
|
get_user_pages()). Note: flush_dcache_page() deliberately
|
|
doesn't work for an anonymous page. The default
|
|
implementation is a nop (and should remain so for all coherent
|
|
architectures). For incoherent architectures, it should flush
|
|
the cache of the page at vmaddr.
|
|
|
|
void flush_kernel_dcache_page(struct page *page)
|
|
When the kernel needs to modify a user page is has obtained
|
|
with kmap, it calls this function after all modifications are
|
|
complete (but before kunmapping it) to bring the underlying
|
|
page up to date. It is assumed here that the user has no
|
|
incoherent cached copies (i.e. the original page was obtained
|
|
from a mechanism like get_user_pages()). The default
|
|
implementation is a nop and should remain so on all coherent
|
|
architectures. On incoherent architectures, this should flush
|
|
the kernel cache for page (using page_address(page)).
|
|
|
|
|
|
void flush_icache_range(unsigned long start, unsigned long end)
|
|
When the kernel stores into addresses that it will execute
|
|
out of (eg when loading modules), this function is called.
|
|
|
|
If the icache does not snoop stores then this routine will need
|
|
to flush it.
|
|
|
|
void flush_icache_page(struct vm_area_struct *vma, struct page *page)
|
|
All the functionality of flush_icache_page can be implemented in
|
|
flush_dcache_page and update_mmu_cache. In the future, the hope
|
|
is to remove this interface completely.
|
|
|
|
The final category of APIs is for I/O to deliberately aliased address
|
|
ranges inside the kernel. Such aliases are set up by use of the
|
|
vmap/vmalloc API. Since kernel I/O goes via physical pages, the I/O
|
|
subsystem assumes that the user mapping and kernel offset mapping are
|
|
the only aliases. This isn't true for vmap aliases, so anything in
|
|
the kernel trying to do I/O to vmap areas must manually manage
|
|
coherency. It must do this by flushing the vmap range before doing
|
|
I/O and invalidating it after the I/O returns.
|
|
|
|
void flush_kernel_vmap_range(void *vaddr, int size)
|
|
flushes the kernel cache for a given virtual address range in
|
|
the vmap area. This is to make sure that any data the kernel
|
|
modified in the vmap range is made visible to the physical
|
|
page. The design is to make this area safe to perform I/O on.
|
|
Note that this API does *not* also flush the offset map alias
|
|
of the area.
|
|
|
|
void invalidate_kernel_vmap_range(void *vaddr, int size) invalidates
|
|
the cache for a given virtual address range in the vmap area
|
|
which prevents the processor from making the cache stale by
|
|
speculatively reading data while the I/O was occurring to the
|
|
physical pages. This is only necessary for data reads into the
|
|
vmap area.
|