linux-kernel - Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle

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Date:   Wed, 18 May 2022 10:27:37 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-pm@...r.kernel.org
Cc:     tglx@...utronix.de, dave.hansen@...ux.intel.com,
        peterz@...radead.org, bp@...en8.de, rafael@...nel.org,
        ravi.v.shankar@...el.com
Subject: Re: [PATCH v4 1/2] x86/fpu: Add a helper to prepare AMX state for
 low-power CPU idle

On 5/18/22 10:20, Chang S. Bae wrote:
>> That also has a nice side effect that non-AMX systems will get to use a
>> static branch and can also skip over the XGETBV1 entirely.
> 
> Yes, but FWIW, as it is non-architectural, the function should be
> consumed only by drivers for AMX systems.

Oh, that's a good point.  The new flag is set only on Sapphire Rapids
systems so the new code is only called there as well.  I guess it would
only matter if we end up having systems that need this where AMX isn't
universally available, like if it were fused on or off on certain SKUs
(I have no idea if anyone is actually doing this).