netdev - Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST

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Message-ID: <20190408174805.GJ25880@linux.intel.com>
Date:   Mon, 8 Apr 2019 10:48:05 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Michael Chan <michael.chan@...adcom.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL

On Mon, Apr 08, 2019 at 05:54:25PM +0800, Xiaoyao Li wrote:
> On Fri, 2019-04-05 at 14:30 +0200, Thomas Gleixner wrote:
> > On Wed, 3 Apr 2019, Fenghua Yu wrote:
> > > @@ -1663,6 +1663,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct
> > > msr_data *msr_info)
> > >  	u32 index;
> > >  
> > >  	switch (msr_info->index) {
> > > +	case MSR_TEST_CTL:
> > > +		if (!msr_info->host_initiated &&
> > > +		    !(vcpu->arch.core_capability & CORE_CAP_SPLIT_LOCK_DETECT))
> > 
> > Errm? If the MSR_TEST_CTL is exposed to the guest via CPUID then the
> > rdmsr() in the guest is not supposed to fail just because
> > CORE_CAP_SPLIT_LOCK_DETECT is not set.
> 
> you're right.
> 
> > vmx->msr_test_ctl holds the proper value, which is either 0 or
> > CORE_CAP_SPLIT_LOCK_DETECT until more bits are supported.
> > 
> > So the chek needs to be guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY).
> 
> I don't think so. There is no dependecy between
> guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY) and MSR_TEST_CTL.
> guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY) only indicates that guest has MSR
> CORE_CAPABILITY.
> 
> Due to the fact that MSR_TEST_CTL is emulated with vmx->msr_test_ctl. I think it
> 's ok to always let userspace or guest to read MSR_TEST_CTL, it just returns the
> emulated value. Like you said, " vmx->msr_test_ctl holds the proper value, which
> is either 0 or CORE_CAP_SPLIT_LOCK_DETECT until more bits are supported."

Assuming the next version implements "vmx->msr_test_ctl_mask", KVM should
inject #GP if the guest attempts RDMSR(MSR_TEST_CTL) and the mask is zero.
It stands to reason that a kernel can only reasonably assume the MSR exists
if the (virtual) CPU supports at least one feature enabled via MSR_TEST_CTL.