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

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Message-ID: <6cdba263-889f-ce98-b7da-4a1380cedc65@intel.com>
Date:   Wed, 16 Jun 2021 12:28:54 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andy Lutomirski <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 6/16/21 12:23 PM, Bae, Chang Seok wrote:
> On Jun 16, 2021, at 12:01, Hansen, Dave <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.
> This was checked to convince the benefit intended by PATCH25 --
> https://lore.kernel.org/lkml/20210523193259.26200-26-chang.seok.bae@intel.com/

That's odd.  How is it possible that the performance of XGETBV(1)
informed the design of that patch without there being any mention of
XGETBV in the comments or changelog?