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

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Message-ID: <4411de99-e827-6119-394b-b994131d6554@linux.intel.com>
Date:   Mon, 28 Jun 2021 18:11:38 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "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 6/17/2021 3:28 AM, Dave Hansen wrote:
> 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?
Hi Chang,

I noticed the XGETBV(1) cycles you ran, however I calculated only ~16 
cycles
in the corresponding machine.

BRs,
Jing