- /* 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.