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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Date:   Mon, 29 Mar 2021 10:43:05 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Len Brown <lenb@...nel.org>
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 Mar 29, 2021, at 9:06 AM, Len Brown <lenb@...nel.org> wrote:
> 
> On Mon, Mar 29, 2021 at 11:43 AM Len Brown <lenb@...nel.org> wrote:
>> 
>> On Mon, Mar 29, 2021 at 9:33 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> 
>>>> I found the author of this passage, and he agreed to revise it to say this
>>>> was targeted primarily at VMMs.
>>> 
>>> Why would this only a problem for VMMs?
>> 
>> VMMs may have to emulate different hardware for different guest OS's,
>> and they would likely "context switch" XCR0 to achieve that.
>> 
>> As switching XCR0 at run-time would confuse the heck out of user-space,
>> it was not imagined that a bare-metal OS would do that.
> 
> to clarify...
> *switching* XCR0 on context switch is slow, but perfectly legal.

How slow is it?  And how slow is switching XFD?  XFD is definitely serializing?