From:   Thomas Gleixner <tglx@linutronix.de>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Yang Zhong <yang.zhong@intel.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com
Cc:     seanjc@google.com, jun.nakajima@intel.com, kevin.tian@intel.com,
        jing2.liu@linux.intel.com, jing2.liu@intel.com
Subject: Re: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly
In-Reply-To: <87c26050-9242-e6fb-3fce-b6bde815f76a@redhat.com>
References: <20211208000359.2853257-1-yang.zhong@intel.com>
 <20211208000359.2853257-16-yang.zhong@intel.com> <87pmq4vw54.ffs@tglx>
 <87c26050-9242-e6fb-3fce-b6bde815f76a@redhat.com>
Date:   Sat, 11 Dec 2021 14:10:52 +0100
Message-ID: <8735mzwalf.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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.