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

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Message-ID: <CAJvTdKkZU3kFiC-5_xwVUNnwzM2QmVS3u1aY-RY=S6_Tw3f5zg@mail.gmail.com>
Date:   Mon, 29 Mar 2021 14:57:25 -0400
From:   Len Brown <lenb@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [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@...capital.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