Re: [PATCH v12 07/11] KVM: vmx/pmu: Unmask LBR fields in the MSR_IA32_DEBUGCTLMSR emualtion
From: Sean Christopherson
Date: Tue Jul 07 2020 - 16:21:57 EST
On Sat, Jun 13, 2020 at 05:42:50PM +0800, Xu, Like wrote:
> On 2020/6/13 17:14, Xiaoyao Li wrote:
> >On 6/13/2020 4:09 PM, Like Xu wrote:
> >>When the LBR feature is reported by the vmx_get_perf_capabilities(),
> >>the LBR fields in the [vmx|vcpu]_supported debugctl should be unmasked.
> >>
> >>The debugctl msr is handled separately in vmx/svm and they're not
> >>completely identical, hence remove the common msr handling code.
I would prefer to put the "remove DEBUGCTRL handling from common x86" in a
separate patch. Without digging into SVM, it's not obvious that dropping
MSR_IA32_DEBUGCTLMSR from kvm_set_msr_common() is a nop for SVM.
> >>Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> >>---
> >> arch/x86/kvm/vmx/capabilities.h | 12 ++++++++++++
> >> arch/x86/kvm/vmx/pmu_intel.c | 19 +++++++++++++++++++
> >> arch/x86/kvm/x86.c | 13 -------------
> >> 3 files changed, 31 insertions(+), 13 deletions(-)
> >>
> >>diff --git a/arch/x86/kvm/vmx/capabilities.h
> >>b/arch/x86/kvm/vmx/capabilities.h
> >>index b633a90320ee..f6fcfabb1026 100644
> >>--- a/arch/x86/kvm/vmx/capabilities.h
> >>+++ b/arch/x86/kvm/vmx/capabilities.h
> >>@@ -21,6 +21,8 @@ extern int __read_mostly pt_mode;
> >> #define PMU_CAP_FW_WRITES (1ULL << 13)
> >> #define PMU_CAP_LBR_FMT 0x3f
> >> +#define DEBUGCTLMSR_LBR_MASK (DEBUGCTLMSR_LBR |
> >>DEBUGCTLMSR_FREEZE_LBRS_ON_PMI)
> >>+
> >> struct nested_vmx_msrs {
> >> /*
> >> * We only store the "true" versions of the VMX capability MSRs. We
> >>@@ -387,4 +389,14 @@ static inline u64 vmx_get_perf_capabilities(void)
> >> return perf_cap;
> >> }
> >> +static inline u64 vmx_get_supported_debugctl(void)
> >>+{
> >>+ u64 val = 0;
> >>+
> >>+ if (vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT)
> >>+ val |= DEBUGCTLMSR_LBR_MASK;
> >>+
> >>+ return val;
> >>+}
> >>+
> >> #endif /* __KVM_X86_VMX_CAPS_H */
> >>diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> >>index a953c7d633f6..d92e95b64c74 100644
> >>--- a/arch/x86/kvm/vmx/pmu_intel.c
> >>+++ b/arch/x86/kvm/vmx/pmu_intel.c
> >>@@ -187,6 +187,7 @@ 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_IA32_DEBUGCTLMSR:
> >> case MSR_IA32_PERF_CAPABILITIES:
> >> ret = 1;
> >> break;
> >>@@ -237,6 +238,9 @@ static int intel_pmu_get_msr(struct kvm_vcpu *vcpu,
> >>struct msr_data *msr_info)
> >> return 1;
> >> msr_info->data = vcpu->arch.perf_capabilities;
> >> return 0;
> >>+ case MSR_IA32_DEBUGCTLMSR:
> >>+ msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> >
> >Can we put the emulation of MSR_IA32_DEBUGCTLMSR in vmx_{get/set})_msr().
> >AFAIK, MSR_IA32_DEBUGCTLMSR is not a pure PMU related MSR that there is
> >bit 2 to enable #DB for bus lock.
> We already have "case MSR_IA32_DEBUGCTLMSR" handler in the vmx_set_msr()
> and you may apply you bus lock changes in that handler.
Hrm, but that'd be weird dependency as vmx_set_msr() would need to check for
#DB bus lock support but not actually write GUEST_IA32_DEBUGCTL, or we'd end
up writing it twice when both bus lock and LBR are supported.
I don't see anything in the series that takes action on writes to
MSR_IA32_DEBUGCTLMSR beyond updating the VMCS, i.e. AFAICT there isn't any
reason to call into the PMU, VMX can simply query vmx_get_perf_capabilities()
to check if it's legal to enable DEBUGCTLMSR_LBR_MASK.
A question for both LBR and bus lock: would it make sense to cache the
guest's value in vcpu_vmx so that querying the guest value doesn't require
a VMREAD? I don't have a good feel for how frequently it would be accessed.
> >>+ return 0;
> >> default:
> >> if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> >> (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> >>@@ -282,6 +286,16 @@ static inline bool lbr_is_compatible(struct
> >>kvm_vcpu *vcpu)
> >> return true;
> >> }
> >> +static inline u64 vcpu_get_supported_debugctl(struct kvm_vcpu *vcpu)
> >>+{
> >>+ u64 debugctlmsr = vmx_get_supported_debugctl();
> >>+
> >>+ if (!lbr_is_enabled(vcpu))
> >>+ debugctlmsr &= ~DEBUGCTLMSR_LBR_MASK;
> >>+
> >>+ return debugctlmsr;
> >>+}
> >>+
> >> static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data
> >>*msr_info)
> >> {
> >> struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> >>@@ -336,6 +350,11 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu,
> >>struct msr_data *msr_info)
> >> }
> >> vcpu->arch.perf_capabilities = data;
> >> return 0;
> >>+ case MSR_IA32_DEBUGCTLMSR:
> >>+ if (data & ~vcpu_get_supported_debugctl(vcpu))
> >>+ return 1;
> >>+ vmcs_write64(GUEST_IA32_DEBUGCTL, data);
> >>+ return 0;
> >> default:
> >> if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> >> (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> >>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >>index 00c88c2f34e4..56f275eb4554 100644
> >>--- a/arch/x86/kvm/x86.c
> >>+++ b/arch/x86/kvm/x86.c
> >>@@ -2840,18 +2840,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu,
> >>struct msr_data *msr_info)
> >> return 1;
> >> }
> >> break;
> >>- case MSR_IA32_DEBUGCTLMSR:
> >>- if (!data) {
> >>- /* We support the non-activated case already */
> >>- break;
> >>- } else if (data & ~(DEBUGCTLMSR_LBR | DEBUGCTLMSR_BTF)) {
> >
> >So after this patch, guest trying to set bit DEBUGCTLMSR_BTF will get a
> >#GP instead of being ignored and printing a log in kernel.
> >
>
> Since the BTF is not implemented on the KVM at all,
> I do propose not left this kind of dummy thing in the future KVM code.
>
> Let's see if Netware or any BTF user will complain about this change.
If you want to drop that behavior it needs be done in a separate patch.
Personally I don't see the point in doing so, it's a trivial amount of code
in KVM and there's no harm in dropping the bits on write.