linux-kernel - Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

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Date:   Tue, 29 Jun 2021 10:54:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state

On 6/29/21 10:43 AM, Bae, Chang Seok wrote:
> On Jun 16, 2021, at 12:01, Dave Hansen <dave.hansen@...el.com> wrote:
>> On 6/16/21 11:47 AM, Bae, Chang Seok wrote:
>>> Reading XINUSE via XGETBV is cheap but not free. I don't know spending a
>>> hundred cycles for this WARN is big deal but this is one of the most
>>> performance-critical paths.
>> Is XGETBV(1) really a hundred cycles?  That seems absurdly high for a
>> non-serializing register read.
> My apologies -- I neglect not to check the log immediately. XGETBV(1) took
> about 26 cycles on my test.

How about we pay this 26-cycle cost, but only when XFD is in use?  We
could either look at the shadowed value of the XFD MSR, or flip a global
variable the first time XFD gets used.