3.16.61-rc1 review patch. If anyone has any objections, please let me know.
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From: Dan Williams dan.j.williams@intel.com
commit eab6870fee877258122a042bfd99ee7908c40280 upstream.
Mark Rutland noticed that GCC optimization passes have the potential to elide necessary invocations of the array_index_mask_nospec() instruction sequence, so mark the asm() volatile.
Mark explains:
"The volatile will inhibit *some* cases where the compiler could lift the array_index_nospec() call out of a branch, e.g. where there are multiple invocations of array_index_nospec() with the same arguments:
if (idx < foo) { idx1 = array_idx_nospec(idx, foo) do_something(idx1); }
< some other code >
if (idx < foo) { idx2 = array_idx_nospec(idx, foo); do_something_else(idx2); }
... since the compiler can determine that the two invocations yield the same result, and reuse the first result (likely the same register as idx was in originally) for the second branch, effectively re-writing the above as:
if (idx < foo) { idx = array_idx_nospec(idx, foo); do_something(idx); }
< some other code >
if (idx < foo) { do_something_else(idx); }
... if we don't take the first branch, then speculatively take the second, we lose the nospec protection.
There's more info on volatile asm in the GCC docs:
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile "
Reported-by: Mark Rutland mark.rutland@arm.com Signed-off-by: Dan Williams dan.j.williams@intel.com Acked-by: Mark Rutland mark.rutland@arm.com Acked-by: Thomas Gleixner tglx@linutronix.de Acked-by: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Fixes: babdde2698d4 ("x86: Implement array_index_mask_nospec") Link: https://lkml.kernel.org/lkml/152838798950.14521.4893346294059739135.stgit@dw... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/x86/include/asm/barrier.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/include/asm/barrier.h +++ b/arch/x86/include/asm/barrier.h @@ -38,7 +38,7 @@ static inline unsigned long array_index_ { unsigned long mask;
- asm ("cmp %1,%2; sbb %0,%0;" + asm volatile ("cmp %1,%2; sbb %0,%0;" :"=r" (mask) :"g"(size),"r" (index) :"cc");