linux-kernel - Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <bd5a5457-edd7-fc1c-2285-c8392a902298@intel.com>
Date:   Mon, 24 May 2021 10:39:51 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Len Brown <lenb@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when
 appropriate

On 5/24/21 10:32 AM, Len Brown wrote:
> On Mon, May 24, 2021 at 10:10 AM Dave Hansen <dave.hansen@...el.com> wrote:
>> On 5/23/21 8:13 PM, Andy Lutomirski wrote:
>>> Can we do this just when going idle?
>> Chang, you might also want to talk with folks that do scheduler
>> performance work (I've cc'd Tim).  I know we're always fighting to trim
>> down the idle and wakeup paths.  There might be no other alternative,
>> but unconditionally forcing an AMX XRSTOR on return from idle might be
>> considered nasty.
> I'm not excited about burdening the generic idle path with a CPU 
> feature specific check that would need to be checked on every idle
> entry.

Me neither.

But, the check itself should be cheap.  A cpu_feature_enabled(AMX) check
will eliminate even the cost of a branch on systems without AMX.  You
could probably even get fancy and also use a static branch that doesn't
get enabled until the first AMX user shows up.