linux-kernel - Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD

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Date:   Sat, 11 Dec 2021 14:10:52 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com
Subject: Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly

On Sat, Dec 11 2021 at 02:31, Paolo Bonzini wrote:
> On 12/11/21 01:10, Thomas Gleixner wrote:
>>      2) When the guest triggers #NM is takes an VMEXIT and the host
>>         does:
>> 
>>                  rdmsrl(MSR_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>> 
>>         injects the #NM and goes on.
>> 
>>      3) When the guest writes to MSR_XFD_ERR it takes an VMEXIT and
>>         the host does:
>> 
>>             vcpu->arch.guest_fpu.xfd_err = msrval;
>>             wrmsrl(MSR_XFD_ERR, msrval);
>
> No wrmsrl here I think, the host value is 0 and should stay so.  Instead 
> the wrmsrl will happen the next time the VCPU loop is entred.

I assumed this can be handled in the fast path, but either way.