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

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Message-ID: <2e2ab02c-b324-e136-924a-0376040163a8@redhat.com>
Date:   Wed, 17 Nov 2021 08:31:17 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Nakajima, Jun" <jun.nakajima@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     "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

On 11/17/21 05:52, Nakajima, Jun wrote:
> If a (creative) guest wants to set XFD[AMX] = 1 for fun while keeping
> AMX state alive without saving the AXM state, it may lose the state
> after VM exit/entry.

I think this should not happen, unless you also document that other 
random events (hypothetically, it could be some other core using AMX?) 
can cause the loss of XTILEDATA if XFD[AMX]=1.  Virtualization should 
not be special, I'd prefer that the guest has the same behavior as bare 
metal in this respect.

Paolo