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

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Message-ID: <3AB6DECB-2A53-4EC2-84A6-0CACE44CFC1C@intel.com>
Date:   Tue, 29 Jun 2021 17:43:02 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Hansen, Dave" <dave.hansen@...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 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.

Thanks,
Chang