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Date:   Thu, 14 Oct 2021 13:39:29 +0200
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Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
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To:     "Liu, Jing2" <jing2.liu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     "x86@kernel.org" <x86@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        Jing Liu <jing2.liu@linux.intel.com>,
        "seanjc@google.com" <seanjc@google.com>,
        "Cooper, Andrew" <andrew.cooper3@citrix.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
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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