linux-kernel - RE: [PATCH v3 22/22] kvm: x86: Disable interception for IA32

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Message-ID: <BN9PR11MB5276B64A8F28FF23A71C949A8C449@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 29 Dec 2021 07:37:14 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Liu, Jing2" <jing2.liu@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
Subject: RE: [PATCH v3 22/22] kvm: x86: Disable interception for IA32_XFD on
 demand

> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Wednesday, December 29, 2021 9:05 AM
> > +	if (vcpu->arch.xfd_out_of_sync)
> 
> Rather than adding a flag that tracks whether or not the MSR can be written
> by
> the guest, can't this be:
> 
> 	if (!vmx_test_msr_bitmap_write(vcpu->loaded_vmcs->msr_bitmap))
> 		fpu_sync_guest_vmexit_xfd_state();
> 

and forgot to mention a change different from above. It's in x86
common exit path but above is vmx specific check. I'm not sure
whether it's worthy of introducing another kvm_x86_ops callback
just for this minor usage.

Introducing an extra flag (e.g. vcpu->arch.xfd_disable_interception)
sounds simpler here.

Thanks
Kevin