Re: [PATCH] KVM: x86: Fix the initial value of mcg_cap

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 20, 2022, Xiaoyao Li wrote:
> On 10/20/2022 10:27 PM, Sean Christopherson wrote:
> > On Thu, Oct 20, 2022, Xiaoyao Li wrote:
> > > vcpu->arch.mcg_cap represents the value of MSR_IA32_MCG_CAP. It's
> > > set via ioctl(KVM_X86_SETUP_MCE) from userspace when exposing and
> > > configuring MCE to guest.
> > > 
> > > It's wrong to leave the default value as KVM_MAX_MCE_BANKS.
> > 
> > Why?  I agree it's an odd default, but the whole MCE API is odd.  Functionally,
> > I don't see anything that's broken by allowing the guest to access the MCx_CTL MSRs
> > by default.
> 
> Yes. Allowing the access doesn't cause any issue for a VM.
> 
> However, for the perspective of virtualization. It virtualizes a magic
> hardware that even CPUID.MCA/MCE is not advertised and MCE is not set up by
> userspace, guest is told there are 32 banks and all the banks can be
> accessed.

'0' isn't necessarily better though, e.g. if userspace parrots back KVM's "supported"
CPUID without invoking KVM_X86_SETUP_MCE, then it's equally odd that the guest will
see no supported MCE MSRS.  

Older versions of the SDM also state (or at least very strongly imply) that banks
0-3 are always available on P6.

Bank 0 is an especially weird case, as several of the MSRs are aliased to other
MSRs that predate the machine check architecture.

Anyways, if this were newly introduced code I'd be all for defaulting to '0', but
KVM has defaulted to KVM_MAX_MCE_BANKS since KVM_X86_SETUP_MCE was added way back
in 2009.  Unless there's a bug that's fixed by this, I'm inclined to keep the
current behavior even though it's weird, as hiding all MCE MSRs by default could
theoretically cause a regression, e.g. by triggering #GP on MSRs that an older
guest expects to always exist.

If we really want to clean up this code, I think the correct approach would be to
inject #GP on all relevant MSRs if CPUID.MCA==0, e.g.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bd5f8a751de..97fafd851d8d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3260,6 +3260,9 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
        u64 data = msr_info->data;
        u32 offset, last_msr;
 
+       if (!msr_info->host_initiated && !guest_cpuid_has(X86_FEATURE_MCA))
+               return 1;
+
        switch (msr) {
        case MSR_IA32_MCG_STATUS:
                vcpu->arch.mcg_status = data;
@@ -3891,6 +3894,14 @@ static int get_msr_mce(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
        unsigned bank_num = mcg_cap & 0xff;
        u32 offset, last_msr;
 
+       if (msr == MSR_IA32_P5_MC_ADDR || msr == MSR_IA32_P5_MC_TYPE) {
+               *pdata = 0;
+               return 0;
+       }
+
+       if (!host && !guest_cpuid_has(X86_FEATURE_MCA))
+               return 1;
+
        switch (msr) {
        case MSR_IA32_P5_MC_ADDR:
        case MSR_IA32_P5_MC_TYPE:

Or alternatively, this should work too:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bd5f8a751de..e4a44d7af0a6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3774,6 +3774,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
        case MSR_IA32_MCG_STATUS:
        case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
        case MSR_IA32_MC0_CTL2 ... MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) - 1:
+               if (!msr_info->host_initiated &&
+                   !guest_cpuid_has(X86_FEATURE_MCA))
+                       return 1;
                return set_msr_mce(vcpu, msr_info);
 
        case MSR_K7_PERFCTR0 ... MSR_K7_PERFCTR3:
@@ -4142,13 +4145,17 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 
                msr_info->data = vcpu->arch.msr_kvm_poll_control;
                break;
-       case MSR_IA32_P5_MC_ADDR:
-       case MSR_IA32_P5_MC_TYPE:
        case MSR_IA32_MCG_CAP:
        case MSR_IA32_MCG_CTL:
        case MSR_IA32_MCG_STATUS:
        case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
        case MSR_IA32_MC0_CTL2 ... MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) - 1:
+               if (!msr_info->host_initiated &&
+                   !guest_cpuid_has(X86_FEATURE_MCA))
+                       return 1;
+               fallthrough;
+       case MSR_IA32_P5_MC_ADDR:
+       case MSR_IA32_P5_MC_TYPE:
                return get_msr_mce(vcpu, msr_info->index, &msr_info->data,
                                   msr_info->host_initiated);
        case MSR_IA32_XSS: