Re: [PATCH v8 13/14] KVM/x86/vPMU: check the lbr feature before entering guest

[Wei Wang]

On 08/06/2019 03:16 PM, Wei Wang wrote:
> The guest can access the lbr related msrs only when the vcpu's lbr event
> has been assigned the lbr feature. A cpu pinned lbr event (though no such
> event usages in the current upstream kernel) could reclaim the lbr feature
> from the vcpu's lbr event (task pinned) via ipi calls. If the cpu is
> running in the non-root mode, this will cause the cpu to vm-exit to handle
> the host ipi and then vm-entry back to the guest. So on vm-entry (where
> interrupt has been disabled), we double confirm that the vcpu's lbr event
> is still assigned the lbr feature via checking event->oncpu.
>
> The pass-through of the lbr related msrs will be cancelled if the lbr is
> reclaimed, and the following guest accesses to the lbr related msrs will
> vm-exit to the related msr emulation handler in kvm, which will prevent
> the accesses.
>
> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> ---
>   arch/x86/kvm/pmu.c           |  6 ++++++
>   arch/x86/kvm/pmu.h           |  3 +++
>   arch/x86/kvm/vmx/pmu_intel.c | 35 +++++++++++++++++++++++++++++++++++
>   arch/x86/kvm/x86.c           | 13 +++++++++++++
>   4 files changed, 57 insertions(+)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index afad092..ed10a57 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -339,6 +339,12 @@ bool kvm_pmu_lbr_enable(struct kvm_vcpu *vcpu)
>   	return false;
>   }
>   
> +void kvm_pmu_enabled_feature_confirm(struct kvm_vcpu *vcpu)
> +{
> +	if (kvm_x86_ops->pmu_ops->enabled_feature_confirm)
> +		kvm_x86_ops->pmu_ops->enabled_feature_confirm(vcpu);
> +}
> +
>   void kvm_pmu_deliver_pmi(struct kvm_vcpu *vcpu)
>   {
>   	if (lapic_in_kernel(vcpu))
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index f875721..7467907 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -30,6 +30,7 @@ struct kvm_pmu_ops {
>   	int (*is_valid_msr_idx)(struct kvm_vcpu *vcpu, unsigned idx);
>   	bool (*is_valid_msr)(struct kvm_vcpu *vcpu, u32 msr);
>   	bool (*lbr_enable)(struct kvm_vcpu *vcpu);
> +	void (*enabled_feature_confirm)(struct kvm_vcpu *vcpu);
>   	int (*get_msr)(struct kvm_vcpu *vcpu, struct msr_data *msr_info);
>   	int (*set_msr)(struct kvm_vcpu *vcpu, struct msr_data *msr_info);
>   	void (*sched_in)(struct kvm_vcpu *vcpu, int cpu);
> @@ -126,6 +127,8 @@ int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp);
>   
>   bool is_vmware_backdoor_pmc(u32 pmc_idx);
>   
> +void kvm_pmu_enabled_feature_confirm(struct kvm_vcpu *vcpu);
> +
>   extern struct kvm_pmu_ops intel_pmu_ops;
>   extern struct kvm_pmu_ops amd_pmu_ops;
>   #endif /* __KVM_X86_PMU_H */
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 5580f1a..421051aa 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -781,6 +781,40 @@ static void intel_pmu_reset(struct kvm_vcpu *vcpu)
>   	intel_pmu_free_lbr_event(vcpu);
>   }
>   
> +void intel_pmu_lbr_confirm(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +
> +	/*
> +	 * Either lbr_event being NULL or lbr_used being false indicates that
> +	 * the lbr msrs haven't been passed through to the guest, so no need
> +	 * to cancel passthrough.
> +	 */
> +	if (!pmu->lbr_event || !pmu->lbr_used)
> +		return;
> +
> +	/*
> +	 * The lbr feature gets reclaimed via IPI calls, so checking of
> +	 * lbr_event->oncpu needs to be in an atomic context. Just confirm
> +	 * that irq has been disabled already.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * Cancel the pass-through of the lbr msrs if lbr has been reclaimed
> +	 * by the host perf.
> +	 */
> +	if (pmu->lbr_event->oncpu != -1) {

A mistake here,  should be "pmu->lbr_event->oncpu == -1".
(It didn't seem to affect the profiling result, but generated
more vm-exits due to mistakenly cancelling the passthrough)

Best,
Wei