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Re: [Xen-devel] [Patch 2/4] Refining Xsave/Xrestore support

>>> On 28.10.10 at 10:33, Haitao Shan <maillists.shan@xxxxxxxxx> wrote:
> I once considered this problem, too. Originally, I suppose I can use
> CR4 switching when entering/leaving PV guest, as what HW is doing for
> us in VMX. But I suspect this will bring a lot of overhead.
> Later I picked the current implementation, hoping guest applications
> can have a check on XCR0 first before it uses extended states. And
> according to spec, in order to use extend states, XCR0 must be set,
> but this is a ring 0 instruction only.

Ah, yes, that's correct of course. So I guess pv-ops could use the
CPUID.OSXSAVE approach then, too.


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