Re: [PATCH v7 1/3] KVM: x86: Provide per VM capability for disabling PMU virtualization

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 15, 2022, David Dunn wrote:
> Add a new capability, KVM_CAP_PMU_CAPABILITY, that takes a bitmask of
> settings/features to allow userspace to configure PMU virtualization on
> a per-VM basis.  For now, support a single flag, KVM_CAP_PMU_DISABLE,
> to allow disabling PMU virtualization for a VM even when KVM is configured
> with enable_pmu=true a module level.
> 
> To keep KVM simple, disallow changing VM's PMU configuration after vCPUs
> have been created.
> 
> Signed-off-by: David Dunn <daviddunn@google.com>
> ---

A few nits, otherwise

Reviewed-by: Sean Christopherson <seanjc@google.com>

Somewhat off-topic, looking at this got me looking at vmx_get_perf_capabilities().
We really should cache the result of its RDMSR.

And I believe emulation of WRMSR from the guest to MSR_IA32_PERF_CAPABILITIES is
missing a guest_cpuid_has() check.

>  Documentation/virt/kvm/api.rst  | 22 ++++++++++++++++++++++
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/svm/pmu.c          |  2 +-
>  arch/x86/kvm/vmx/pmu_intel.c    |  2 +-
>  arch/x86/kvm/x86.c              | 17 +++++++++++++++++
>  include/uapi/linux/kvm.h        |  3 +++
>  tools/include/uapi/linux/kvm.h  |  3 +++
>  7 files changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a4267104db50..df836965b347 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7561,3 +7561,25 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
>  of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
>  the hypercalls whose corresponding bit is in the argument, and return
>  ENOSYS for the others.
> +
> +8.35 KVM_CAP_PMU_CAPABILITY
> +---------------------------
> +
> +:Capability KVM_CAP_PMU_CAPABILITY
> +:Architectures: x86
> +:Type: vm
> +:Parameters: arg[0] is bitmask of PMU virtualization capabilities.

Referring to "DISABLE" as a capabilitiy is rather odd, but I don't have a better
suggestion.

> +:Returns 0 on success, -EINVAL when arg[0] contains invalid bits
> +
> +This capability alters PMU virtualization in KVM.
> +
> +Calling KVM_CHECK_EXTENSION for this capability returns a bitmask of
> +PMU virtualization capabilities that can be adjusted on a VM.
> +
> +The argument to KVM_ENABLE_CAP is also a bitmask and selects specific
> +PMU virtualization capabilities to be applied to the VM.  This can
> +only be invoked on a VM prior to the creation of VCPUs.
> +
> +At this time, KVM_CAP_PMU_DISABLE is the only capability.  Setting
> +this capability will disable PMU virtualization for that VM.  Usermode
> +should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 10815b672a26..61e9050a3488 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1233,6 +1233,7 @@ struct kvm_arch {
>  	hpa_t	hv_root_tdp;
>  	spinlock_t hv_root_tdp_lock;
>  #endif
> +	bool enable_pmu;

Doesn''t really matter, but I'd prefer we put this up among the many other bools, e.g.

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4bb0d3be2055..b57d40419ddd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1150,6 +1150,7 @@ struct kvm_arch {
        bool guest_can_read_msr_platform_info;
        bool exception_payload_enabled;
 
+       bool enable_pmu;
        bool bus_lock_detection_enabled;
        /*
         * If exit_on_emulation_error is set, and the in-kernel instruction
@@ -1233,7 +1234,6 @@ struct kvm_arch {
        hpa_t   hv_root_tdp;
        spinlock_t hv_root_tdp_lock;
 #endif
-       bool enable_pmu;
 };
 
 struct kvm_vm_stat {

>  };
>  
>  struct kvm_vm_stat {
> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> index 5aa45f13b16d..d4de52409335 100644
> --- a/arch/x86/kvm/svm/pmu.c
> +++ b/arch/x86/kvm/svm/pmu.c
> @@ -101,7 +101,7 @@ static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
>  {
>  	struct kvm_vcpu *vcpu = pmu_to_vcpu(pmu);
>  
> -	if (!enable_pmu)
> +	if (!vcpu->kvm->arch.enable_pmu)
>  		return NULL;
>  
>  	switch (msr) {
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 03fab48b149c..4e5b1eeeb77c 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -487,7 +487,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>  	pmu->reserved_bits = 0xffffffff00200000ull;
>  
>  	entry = kvm_find_cpuid_entry(vcpu, 0xa, 0);
> -	if (!entry || !enable_pmu)
> +	if (!entry || !vcpu->kvm->arch.enable_pmu)
>  		return;
>  	eax.full = entry->eax;
>  	edx.full = entry->edx;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index eaa3b5b89c5e..0803a1388bbf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -110,6 +110,8 @@ static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
>  
>  #define KVM_EXIT_HYPERCALL_VALID_MASK (1 << KVM_HC_MAP_GPA_RANGE)
>  
> +#define KVM_CAP_PMU_VALID_MASK KVM_CAP_PMU_DISABLE
> +
>  #define KVM_X2APIC_API_VALID_FLAGS (KVM_X2APIC_API_USE_32BIT_IDS | \
>                                      KVM_X2APIC_API_DISABLE_BROADCAST_QUIRK)
>  
> @@ -4331,6 +4333,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		if (r < sizeof(struct kvm_xsave))
>  			r = sizeof(struct kvm_xsave);
>  		break;
> +	case KVM_CAP_PMU_CAPABILITY:
> +		r = enable_pmu ? KVM_CAP_PMU_VALID_MASK : 0;
> +		break;
>  	}
>  	default:
>  		break;
> @@ -6005,6 +6010,17 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		kvm->arch.exit_on_emulation_error = cap->args[0];
>  		r = 0;
>  		break;
> +	case KVM_CAP_PMU_CAPABILITY:
> +		r = -EINVAL;
> +		if (!enable_pmu || (cap->args[0] & ~KVM_CAP_PMU_VALID_MASK))
> +			break;

I'd prefer another newline here, the other case statements are just bad influences :-)

> +		mutex_lock(&kvm->lock);
> +		if (!kvm->created_vcpus) {
> +			kvm->arch.enable_pmu = !(cap->args[0] & KVM_CAP_PMU_DISABLE);
> +			r = 0;
> +		}
> +		mutex_unlock(&kvm->lock);
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;