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Message-Id: <09AB34BD-7DB8-4DBD-A538-F3AE642F8C8A@amacapital.net>
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?
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