linux-kernel - Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

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Message-ID: <9c2e202b-4e15-2728-4c61-a2f74adac444@redhat.com>
Date:   Thu, 14 Oct 2021 13:39:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Liu, Jing2" <jing2.liu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

On 14/10/21 13:30, Liu, Jing2 wrote:
> I guess we're worrying about is when KVM is sched_out, a nonzero XFD_ERR
> can be lost by other host thread. We can save guest XFD_ERR in sched_out
> and restore before next vmenter. Kernel is assumed not using AMX thus
> softirq won't make it lost.
> I think this solves the problem. So we can directly passthrough RW of it,
> and no need to rdmsr(XFD_ERR) in vmexit.

Correct; you can also use the "user-return MSRs" machinery (until Linux 
starts using AMX in the kernel, but that shouldn't happen too soon).

Paolo