linux-kernel - Re: Thoughts of AMX KVM support based on latest kernel

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Message-ID: <87sfvslaha.ffs@tglx>
Date:   Fri, 19 Nov 2021 11:13:05 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Nakajima, Jun" <jun.nakajima@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
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: Thoughts of AMX KVM support based on latest kernel

Jun,

On Thu, Nov 18 2021 at 23:17, Jun Nakajima wrote:
> On Nov 17, 2021, at 4:53 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> It doesn't have to happen in current processors, but it should be
>> architecturally valid behavior to clear the processor's state as soon
>> as a bit in XFD is set to 1.
>
> 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]. It is recommended
> that system software initialize AMX state (e.g., by executing
> TILERELEASE) before doing so. This is because maintaining AMX state in
> a non-initialized state may have negative power and performance
> implications.
>
> System software should not use XFD to implement a “lazy restore”
> approach to management of the XTILEDATA state component. This approach
> will not operate correctly for a variety of reasons. One is that the
> LDTILECFG and TILERELEASE instructions initialize XTILEDATA and do not
> cause an #NM exception. Another is that an execution of XSAVE by a
> user thread will save XTILEDATA as initialized instead of the data
> expected by the user thread.

Can this pretty please be reworded so that it says:

  When setting IA32_XFD[18] the AMX register state is not guaranteed to
  be preserved. The resulting register state depends on the
  implementation.

Also it's a real design disaster that component 17 cannot be fenced off
via XFD. That's really inconsistent and leads exactly to this half
defined state.

Thanks,

        tglx