Re: [PATCH v7 04/15] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_DEPTH for guest Arch LBR

[Yang Weijiang]

On Mon, Aug 09, 2021 at 09:16:47PM +0800, Like Xu wrote:
> On 6/8/2021 3:42 pm, Yang Weijiang wrote:
> >From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> 
> ...
> 
> >
> >The number of Arch LBR entries available is determined by the value
> >in host MSR_ARCH_LBR_DEPTH.DEPTH. The supported LBR depth values are
> >enumerated in CPUID.(EAX=01CH, ECX=0):EAX[7:0]. For each bit "n" set
> >in this field, the MSR_ARCH_LBR_DEPTH.DEPTH value of "8*(n+1)" is
> >supported.
> >
> >On a guest write to MSR_ARCH_LBR_DEPTH, all LBR entries are reset to 0.
> >KVM writes guest requested value to the native ARCH_LBR_DEPTH MSR
> >(this is safe because the two values will be the same) when the Arch LBR
> >records MSRs are pass-through to the guest.
> >
> >Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> >Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> >---
> >  arch/x86/kvm/vmx/pmu_intel.c | 35 ++++++++++++++++++++++++++++++++++-
> >  1 file changed, 34 insertions(+), 1 deletion(-)
> >
> >diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> >index 9efc1a6b8693..a4ef5bbce186 100644
> >--- a/arch/x86/kvm/vmx/pmu_intel.c
> >+++ b/arch/x86/kvm/vmx/pmu_intel.c
> >@@ -211,7 +211,7 @@ static bool intel_pmu_is_valid_lbr_msr(struct kvm_vcpu *vcpu, u32 index)
> >  static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
> >  {
> >  	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> >-	int ret;
> >+	int ret = 0;
> >  	switch (msr) {
> >  	case MSR_CORE_PERF_FIXED_CTR_CTRL:
> >@@ -220,6 +220,10 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
> >  	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
> >  		ret = pmu->version > 1;
> >  		break;
> >+	case MSR_ARCH_LBR_DEPTH:
> >+		if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> >+			ret = guest_cpuid_has(vcpu, X86_FEATURE_ARCH_LBR);
> >+		break;
> >  	default:
> >  		ret = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0) ||
> >  			get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0) ||
> >@@ -348,10 +352,28 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
> >  	return true;
> >  }
> >+/*
> >+ * Check if the requested depth value the same as that of host.
> >+ * When guest/host depth are different, the handling would be tricky,
> >+ * so now only max depth is supported for both host and guest.
> >+ */
> >+static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> >+{
> >+	unsigned int eax, ebx, ecx, edx;
> >+
> >+	if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
> >+		return false;
> >+
> >+	cpuid_count(0x1c, 0, &eax, &ebx, &ecx, &edx);
> 
> I really don't understand why the sanity check of the
> guest lbr depth needs to read the host's cpuid entry and it's pretty slow.
>
This is to address a concern from Jim:
"Does this imply that, when restoring a vCPU, KVM_SET_CPUID2 must be called before
KVM_SET_MSRS, so that arch_lbr_depth_is_valid() knows what to do? Is this documented
anywhere?" 
anyway, setting depth MSR shouldn't be hot path.
 
> KVM has reported the maximum host LBR depth as the only supported value.
> 
> >+
> >+	return (depth == fls(eax & 0xff) * 8);
> >+}
> >+
> >  static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  {
> >  	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> >  	struct kvm_pmc *pmc;
> >+	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
> >  	u32 msr = msr_info->index;
> >  	switch (msr) {
> >@@ -367,6 +389,9 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  	case MSR_CORE_PERF_GLOBAL_OVF_CTRL:
> >  		msr_info->data = pmu->global_ovf_ctrl;
> >  		return 0;
> >+	case MSR_ARCH_LBR_DEPTH:
> >+		msr_info->data = lbr_desc->records.nr;
> >+		return 0;
> >  	default:
> >  		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> >  		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> >@@ -393,6 +418,7 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  {
> >  	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> >  	struct kvm_pmc *pmc;
> >+	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
> >  	u32 msr = msr_info->index;
> >  	u64 data = msr_info->data;
> >@@ -427,6 +453,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  			return 0;
> >  		}
> >  		break;
> >+	case MSR_ARCH_LBR_DEPTH:
> >+		if (!arch_lbr_depth_is_valid(vcpu, data))
> >+			return 1;
> >+		lbr_desc->records.nr = data;
> >+		if (!msr_info->host_initiated)
> >+			wrmsrl(MSR_ARCH_LBR_DEPTH, lbr_desc->records.nr);
> 
> Resetting the host msr here is dangerous,
> what if the guest LBR event doesn't exist or isn't scheduled on?
Hmm, should be vmcs_write to the DEPTH field, thanks for pointing this
out!
> 
> >+		return 0;
> >  	default:
> >  		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> >  		    (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> >