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powerpc/64s/perf: perf interrupt does not have to get_user_pages to access user memory

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read_user_stack_slow that walks user address translation by hand is
only required on hash, because a hash fault can not be serviced from
"NMI" context (to avoid re-entering the hash code) so the user stack
can be mapped into Linux page tables but not accessible by the CPU.

Radix MMU mode does not have this restriction. A page fault failure
would indicate the page is not accessible via get_user_pages either,
so avoid this on radix.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111120151.3150658-1-npiggin@gmail.com
This commit is contained in:
Nicholas Piggin 2020-11-11 22:01:51 +10:00 committed by Michael Ellerman
parent fdcfeaba38
commit 987c426320
2 changed files with 3 additions and 2 deletions
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2
arch/powerpc/perf/callchain.h
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@ -33,7 +33,7 @@ static inline int __read_user_stack(const void __user *ptr, void *ret,
rc = copy_from_user_nofault(ret, ptr, size);
if (IS_ENABLED(CONFIG_PPC64) && rc)
if (IS_ENABLED(CONFIG_PPC64) && !radix_enabled() && rc)
return read_user_stack_slow(ptr, ret, size);
return rc;

3
arch/powerpc/perf/callchain_64.c
View file

@ -21,7 +21,8 @@
/*
* On 64-bit we don't want to invoke hash_page on user addresses from
* interrupt context, so if the access faults, we read the page tables
* to find which page (if any) is mapped and access it directly.
* to find which page (if any) is mapped and access it directly. Radix
* has no need for this so it doesn't use read_user_stack_slow.
*/
int read_user_stack_slow(const void __user *ptr, void *buf, int nb)
{