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To:     Jing Liu <jing2.liu@linux.intel.com>, seanjc@google.com,
        kvm@vger.kernel.org
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 <20210207154256.52850-4-jing2.liu@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch
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Date:   Mon, 8 Feb 2021 11:25:07 +0100
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On 07/02/21 16:42, Jing Liu wrote:
> |In KVM, "guest_fpu" serves for any guest task working on this vcpu 
> during vmexit and vmenter. We provide a pre-allocated guest_fpu space 
> and entire "guest_fpu.state_mask" to avoid each dynamic features 
> detection on each vcpu task. Meanwhile, to ensure correctly 
> xsaves/xrstors guest state, set IA32_XFD as zero during vmexit and vmenter.|

Most guests will not need the whole xstate feature set.  So perhaps you 
  could set XFD to the host value | the guest value, trap #NM if the 
host XFD is zero, and possibly reflect the exception to the guest's XFD 
and XFD_ERR.

In addition, loading the guest XFD MSRs should use the MSR autoload 
feature (add_atomic_switch_msr).

Paolo