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From:   "Bae, Chang Seok" <chang.seok.bae@intel.com>
To:     "Hansen, Dave" <dave.hansen@intel.com>
CC:     Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        "Brown, Len" <len.brown@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state
Thread-Topic: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of
 dynamic user state
Thread-Index: AQHXUAs71FpA6GLTCUWvxGMv7m3G8KsW+QQAgAAdOYCAAAnJgIAAA/+AgAAGI4A=
Date:   Wed, 16 Jun 2021 19:23:45 +0000
Message-ID: <16681A30-59EA-4E35-8A51-CCD403026C92@intel.com>
References: <20210523193259.26200-1-chang.seok.bae@intel.com>
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On Jun 16, 2021, at 12:01, Hansen, Dave <dave.hansen@intel.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.c=
om/=20

> If you're really worried, let's put it under X86_FEATURE_DEBUG_FPU or
> something.

Yes. I will also make sure to include comments for this.

Thanks,
Chang=