linux-kernel - Re: [PATCH v2 23/23] kvm: x86: Disable RDMSR interception of IA32_XFD

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Date:   Tue, 21 Dec 2021 09:48:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Liu, Jing2" <jing2.liu@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...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>
Cc:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...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 v2 23/23] kvm: x86: Disable RDMSR interception of
 IA32_XFD_ERR

On 12/21/21 07:29, Liu, Jing2 wrote:
>>
> Thanks for reviewing the patches.
> 
> If disable unconditionally in vmx_create_vcpu, it means even guest has
> no cpuid, the msr read is passthrough to guest and it can read a value, which
> seems strange, though spec doesn't mention the read behaviour w/o cpuid.
> 
> How about disabling read interception at vmx_vcpu_after_set_cpuid?
> 
> if (boot_cpu_has(X86_FEATURE_XFD) && guest_cpuid_has(vcpu, X86_FEATURE_XFD))
>          vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R, false);

Even better:

	if (boot_cpu_has(X86_FEATURE_XFD))
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R,
					  !guest_cpuid_has(vcpu, X86_FEATURE_XFD));

Thanks,

Paolo