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SubjectRe: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state
On Mon, Mar 29, 2021 at 1:43 PM Andy Lutomirski <luto@amacapital.net> wrote:

> > *switching* XCR0 on context switch is slow, but perfectly legal.
>
> How slow is it? And how slow is switching XFD? XFD is definitely serializing?

XCR0 writes in a VM guest cause a VMEXIT..
XCR writes in a VM guest do not.

I will find out what the relative cost is on bare metal, where VMEXIT
is not an issue.
--
Len Brown, Intel Open Source Technology Center

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