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>

arch/powerpc/kernel/vector.S

@@ -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