linux-kernel - Re: [PATCH v4 15/17] kvm: x86: Report CORE_CAPABILITY on GET_SUPPORTED

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Date:   Tue, 05 Mar 2019 15:03:28 +0800
From:   Xiaoyao Li <xiaoyao.li@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        kvm@...r.kernel.org
Subject: Re: [PATCH v4 15/17] kvm: x86: Report CORE_CAPABILITY on
 GET_SUPPORTED_CPUID

On Mon, 2019-03-04 at 12:14 +0100, Paolo Bonzini wrote:
> On 04/03/19 12:10, Xiaoyao Li wrote:
> > Like you said before, I think we don't need the condition judgment
> > "if(boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))", but to set
> > F(CORE_CAPABILITY)
> > always for guest since MSR_IA32_CORE_CAPABILITY is emulated.
> > 
> > And we should set the right emulated value of MSR_IA32_CORE_CAPABILITY for
> > guest
> > in function kvm_get_core_capability() based on whether
> > boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) just as you commented in the
> > next
> > patch.
> 
> Yes, that would certainly be better.  However, you'd also have to move
> MSR_IA32_CORE_CAPABILITY handling to x86.c, because you'd have to enable
> X86_FEATURE_CORE_CAPABILITY for AMD.
> 
> Paolo

Hi, Paolo

I just notice that F(ARCH_CAPABILITIES) is set unconditionally. However the
handling of MSR_IA32_ARCH_CAPABILITIES only exists with vmx, and the emulation
of this MSR is in vmx->arch_capabilities.
These will cause #GP when guest kernel rdmsr(MSR_IA32_ARCH_CAPABILITES) with AMD
CPU since there is handling for svm.
Maybe what I think is not correct due to my limit knowledge of
MSR_IA32_ARCH_CAPABILITIES and how kernel handles its related features.

If what I said above is true and it's indeed an issue. So based on the fact that
both MSR_IA32_ARCH_CAPABILITIES and MSR_IA32_CORE_CAPABILITY are feature-
enumerating MSR and we emulate them in KVM, there are 2 choices for us to handle
it:
1. we unconditionally set F(ARCH_CAPABILITIES) and F(CORE_CAPABILITY) for guest,
move the emulation of these 2 MSRs to vcpu->arch.***, and move all the handling 
of these 2 MSRs to x86.c.

2. we conditionally set F(ARCH_CAPABILITIES) and F(CORE_CAPABILITY) only if it
is intel CPU. So we just need to emulate these 2 MSRs in vmx->*** for intel CPU.

I prefer option 2 personally for CORE_CAPABILITY since it makes no sense to
expose MSR_IA32_CORE_CAPABILITY to other x86 vendors.
About ARCH_CAPABILITIES, it seems that we emulate it for generic x86 cpus that
!x86_match_cpu(cpu_no_speculation). So we should choose option 1, to move the
emulation and handling to x86.c?

Xiaoyao