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>
diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
index 1c2e7a3..616a6d8 100644
--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S
@@ -70,10 +70,11 @@
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