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To:     Sean Christopherson <seanjc@google.com>
Cc:     Jing Liu <jing2.liu@linux.intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, jing2.liu@intel.com
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch
Message-ID: <77b27707-721a-5c6a-c00d-e1768da55c64@redhat.com>
Date:   Mon, 8 Feb 2021 18:45:35 +0100
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On 08/02/21 18:31, Sean Christopherson wrote:
> On Mon, Feb 08, 2021, Paolo Bonzini wrote:
>> 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).
> 
> Why do you say that?  I would strongly prefer to use the load lists only if they
> are absolutely necessary.  I don't think that's the case here, as I can't
> imagine accessing FPU state in NMI context is allowed, at least not without a
> big pile of save/restore code.

I was thinking more of the added vmentry/vmexit overhead due to 
xfd_guest_enter xfd_guest_exit.

That said, the case where we saw MSR autoload as faster involved EFER, 
and we decided that it was due to TLB flushes (commit f6577a5fa15d, 
"x86, kvm, vmx: Always use LOAD_IA32_EFER if available", 2014-11-12). 
Do you know if RDMSR/WRMSR is always slower than MSR autoload?

Paolo