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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state
Date:   Mon, 29 Mar 2021 10:43:05 -0700
Message-Id: <09AB34BD-7DB8-4DBD-A538-F3AE642F8C8A@amacapital.net>
References: <CAJvTdKm4qKw8D8b+NokBsdiE5yBj=LTiH50VuxJY2aAL8qQq6g@mail.gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        "Brown, Len" <len.brown@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <CAJvTdKm4qKw8D8b+NokBsdiE5yBj=LTiH50VuxJY2aAL8qQq6g@mail.gmail.com>
To:     Len Brown <lenb@kernel.org>
Precedence: bulk


> On Mar 29, 2021, at 9:06 AM, Len Brown <lenb@kernel.org> wrote:
>=20
> =EF=BB=BFOn Mon, Mar 29, 2021 at 11:43 AM Len Brown <lenb@kernel.org> wrot=
e:
>>=20
>> On Mon, Mar 29, 2021 at 9:33 AM Thomas Gleixner <tglx@linutronix.de> wrot=
e:
>>=20
>>>> I found the author of this passage, and he agreed to revise it to say t=
his
>>>> was targeted primarily at VMMs.
>>>=20
>>> Why would this only a problem for VMMs?
>>=20
>> VMMs may have to emulate different hardware for different guest OS's,
>> and they would likely "context switch" XCR0 to achieve that.
>>=20
>> 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.
>=20
> 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 serializi=
ng?