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powerpc: Improve comment explaining why we modify VRSAVE 

The comment explaining why we modify VRSAVE is misleading, glibc
does rely on the behaviour. Update the comment.

Signed-off-by: Anton Blanchard <anton@samba.org>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anton Blanchard 2016-05-20 04:41:34 +10:00 committed by Michael Ellerman
parent 1a1cee843c
commit dd57023747
1 changed files with 5 additions and 4 deletions
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arch/powerpc/kernel/vector.S
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@ -70,10 +70,11 @@ _GLOBAL(load_up_altivec)
MTMSRD(r5) /* enable use of AltiVec now */
isync
/* Hack: if we get an altivec unavailable trap with VRSAVE
* set to all zeros, we assume this is a broken application
* that fails to set it properly, and thus we switch it to
* all 1's
/*
* While userspace in general ignores VRSAVE, glibc uses it as a boolean
* to optimise userspace context save/restore. Whenever we take an
* altivec unavailable exception we must set VRSAVE to something non
* zero. Set it to all 1s. See also the programming note in the ISA.
*/
mfspr r4,SPRN_VRSAVE
cmpwi 0,r4,0