Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

[Len Brown]

On Mon, Mar 29, 2021 at 1:43 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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