linux-kernel - Re: Rewording of Setting IA32_XFD[18] (Re: Thoughts of AMX KVM support based on latest kernel)

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Message-ID: <3cf70fa7-8e3e-5066-bdf4-59aec5215640@redhat.com>
Date:   Wed, 8 Dec 2021 14:57:31 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Nakajima, Jun" <jun.nakajima@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Subject: Re: Rewording of Setting IA32_XFD[18] (Re: Thoughts of AMX KVM
 support based on latest kernel)

On 12/8/21 01:50, Nakajima, Jun wrote:
> 3.3 RECOMMENDATIONS FOR SYSTEM SOFTWARE
> 
> System software may disable use of Intel AMX by clearing XCR0[18:17],
> by clearing CR4.OSXSAVE, or by setting IA32_XFD[18]. System software
> should initialize AMX state (e.g., by executing TILERELEASE) when
> doing so because maintaining AMX state in a non-initialized state may
> have negative power and performance implications. In addition,
> software should not rely on the state of the tile data after setting

I would change this to "must not rely", otherwise looks good.  Thanks!

Paolo

> IA32_XFD[18]; software should always reload or reinitialize the tile
> data after clearing IA32_XFD[18].