Re: [RFC PATCH 41/41] KVM: nVMX: Add nested virtualization support for passthrough PMU

[Sean Christopherson]

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> From: Mingwei Zhang <mizhang@xxxxxxxxxx>
> 
> Add nested virtualization support for passthrough PMU by combining the MSR
> interception bitmaps of vmcs01 and vmcs12. Readers may argue even without
> this patch, nested virtualization works for passthrough PMU because L1 will
> see Perfmon v2 and will have to use legacy vPMU implementation if it is
> Linux. However, any assumption made on L1 may be invalid, e.g., L1 may not
> even be Linux.
> 
> If both L0 and L1 pass through PMU MSRs, the correct behavior is to allow
> MSR access from L2 directly touch HW MSRs, since both L0 and L1 passthrough
> the access.
> 
> However, in current implementation, if without adding anything for nested,
> KVM always set MSR interception bits in vmcs02. This leads to the fact that
> L0 will emulate all MSR read/writes for L2, leading to errors, since the
> current passthrough vPMU never implements set_msr() and get_msr() for any
> counter access except counter accesses from the VMM side.
> 
> So fix the issue by setting up the correct MSR interception for PMU MSRs.
> 
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 52 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index c5ec0ef51ff7..95e1c78152da 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -561,6 +561,55 @@ static inline void nested_vmx_set_intercept_for_msr(struct vcpu_vmx *vmx,
>  						   msr_bitmap_l0, msr);
>  }
>  
> +/* Pass PMU MSRs to nested VM if L0 and L1 are set to passthrough. */
> +static void nested_vmx_set_passthru_pmu_intercept_for_msr(struct kvm_vcpu *vcpu,

Heh, 50 instances of passthrough, and then someone decides to shave a few characters
with passthru :-)  Long live mediated PMU!!!

> +							  unsigned long *msr_bitmap_l1,
> +							  unsigned long *msr_bitmap_l0)
> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	int i;
> +
> +	for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
> +		nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +						 msr_bitmap_l0,
> +						 MSR_ARCH_PERFMON_EVENTSEL0 + i,
> +						 MSR_TYPE_RW);
> +		nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +						 msr_bitmap_l0,
> +						 MSR_IA32_PERFCTR0 + i,
> +						 MSR_TYPE_RW);
> +		nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +						 msr_bitmap_l0,
> +						 MSR_IA32_PMC0 + i,
> +						 MSR_TYPE_RW);
> +	}
> +
> +	for (i = 0; i < vcpu_to_pmu(vcpu)->nr_arch_fixed_counters; i++) {

Curly braces aren't needed, and this can use "pmu" instead of "vcpu_to_pmu".

> +		nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +						 msr_bitmap_l0,
> +						 MSR_CORE_PERF_FIXED_CTR0 + i,
> +						 MSR_TYPE_RW);
> +	}
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +					 msr_bitmap_l0,
> +					 MSR_CORE_PERF_FIXED_CTR_CTRL,
> +					 MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +					 msr_bitmap_l0,
> +					 MSR_CORE_PERF_GLOBAL_STATUS,
> +					 MSR_TYPE_RW);
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +					 msr_bitmap_l0,
> +					 MSR_CORE_PERF_GLOBAL_CTRL,
> +					 MSR_TYPE_RW);
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1,
> +					 msr_bitmap_l0,
> +					 MSR_CORE_PERF_GLOBAL_OVF_CTRL,
> +					 MSR_TYPE_RW);
> +}
> +
>  /*
>   * Merge L0's and L1's MSR bitmap, return false to indicate that
>   * we do not use the hardware.
> @@ -660,6 +709,9 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
>  	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>  					 MSR_IA32_FLUSH_CMD, MSR_TYPE_W);
>  
> +	if (is_passthrough_pmu_enabled(vcpu))
> +		nested_vmx_set_passthru_pmu_intercept_for_msr(vcpu, msr_bitmap_l1, msr_bitmap_l0);

More code that's probably cleaner if the helper handles the PMU type.

	nested_vmx_set_pmu_msr_intercepts(vcpu, msr_bitmap_l1, msr_bitmap_l0);

and then

	if (!enable_mediated_pmu || !pmu->version)
		return;

> +
>  	kvm_vcpu_unmap(vcpu, &vmx->nested.msr_bitmap_map, false);
>  
>  	vmx->nested.force_msr_bitmap_recalc = false;
> -- 
> 2.34.1
>