When submitting this to upstream, the following discussion
turned up that newer kernels would not forcfully set the
bit in CR4 to test for the XSAVE feature but rely on the
cpuid bits passed by the HV. And testing done by Amazon
said they will be fine with that.
(There was a patch mentioned that would mask off the cpuid
bits from guest but I have not linked that to a specific
Xen version. But the hopes is that EC2 does at least use
a version that will do that. And even if they do, they
said they don't need it and we get rid of yet another
Probably we want to drop that in Precise, too. It could allow
guest to use xsave if the HV allows. For Lucid we would need
it still or backport the change in Xen code that replaces the
test for xsave capabilities.