linux-kernel - Re: [PATCH] kvm/x86/vmx: Make the emulation of MSR_IA32_ARCH

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Message-ID: <55caabc3-e075-3282-a000-88356ddb15c2@redhat.com>
Date:   Thu, 7 Mar 2019 19:15:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Xiaoyao Li <xiaoyao.li@...ux.intel.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/x86/vmx: Make the emulation of
 MSR_IA32_ARCH_CAPABILITIES only for vmx

On 07/03/19 18:37, Sean Christopherson wrote:
> On Thu, Mar 07, 2019 at 05:31:43PM +0800, Xiaoyao Li wrote:
>> At present, we report F(ARCH_CAPABILITIES) for x86 arch(both vmx and svm)
>> unconditionally, but we only emulate this MSR in vmx. It will cause #GP
>> while guest kernel rdmsr(MSR_IA32_ARCH_CAPABILITIES) in an AMD host.
>>
>> Since MSR IA32_ARCH_CAPABILITIES is an intel-specific MSR, it makes no
>> sense to emulate it in svm. Thus this patch chooses to only emulate it
>> for vmx, and moves the related handling to vmx related files.
> 
> What about emulating the MSR on an AMD host for testing purpsoes?  It
> might be a useful way for someone without Intel hardware to test spectre
> related flows.
> 
> In other words, an alternative to restricting emulation of the MSR to
> Intel CPUS would be to move MSR_IA32_ARCH_CAPABILITIES handling into
> kvm_{get,set}_msr_common().  Guest access to MSR_IA32_ARCH_CAPABILITIES
> is gated by X86_FEATURE_ARCH_CAPABILITIES in the guest's CPUID, e.g.
> RDMSR will naturally #GP fault if userspace passes through the host's
> CPUID on a non-Intel system.

This is also better because it wouldn't change the guest ABI for AMD
processors.  Dropping CPUID flags is generally not a good idea.

Paolo