Re: [PATCH 12/23] KVM: x86/pmu: Add the perfmon_mask module parameter

[Mi, Dapeng]

I don't remember why to set the perfmon_mask to be a kvm_intel module
parameter, but per my understanding, it seems better and more
straightforward to set perfmon_mask to be a perf/x86 parameter. It's also
consistent with current perf/x86 behavior. Host exposes all PMU
capabilities to KVM instead of the opposite.

Thanks.

On 8/22/2026 6:19 AM, Zide Chen wrote:
> Only Intel CPUs support PerfMon masking, so this new parameter is
> Intel-specific. Mediated vPMU must be enabled for PerfMon masking.
>
> PerfMon masking lets a VMM partition PMU resources between host and
> guest: each mask bit determines whether the guest (set) or host
> (clear) owns a counter's MSR(s), RDPMC access, and the corresponding
> bit in the global MSRs (e.g. IA32_PERF_GLOBAL_CTRL) in non-root mode.
>
> The setting is system-wide and caps what any individual guest may be
> given; each guest may configure a subset of it.
>
> The parameter is configured with a text-based, semicolon-separated
> list of terms. Currently supported for a mediated vPMU guest:
>
>   guest_gp=<list>    general purpose counters (0-based) owned by the guest
>   guest_fixed=<list> fixed counters (0-based) owned by the guest
>   perf_metrics       PERF_METRICS is owned by the guest
>
> For example:
>
>   kvm-intel.perfmon_mask=guest_gp=0-3;guest_fixed=0,2-3;perf_metrics
>
> Internally, the parsed terms are folded into a 64-bit perfmon_mask
> variable that uses the same layout as the PERFMON_MASK VMCS field,
> i.e. the IA32_PERF_GLOBAL_STATUS layout. An empty (default) value
> behaves the same as a plain mediated vPMU.
>
> Using IA32_PERF_GLOBAL_CTRL in the generic VM-exit MSR-store area
> while the PerfMon masking VM-execution control is set is undefined,
> so PerfMon masking requires the dedicated Save-IA32_PERF_GLOBAL_CTRL
> VM-exit control.
>
> cpu_has_vmx_perfmon_mask() is hardcoded to false temporarily until
> later patches.
>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
>  .../admin-guide/kernel-parameters.txt         | 30 ++++++++
>  arch/x86/kvm/pmu.c                            |  3 +
>  arch/x86/kvm/pmu.h                            |  1 +
>  arch/x86/kvm/vmx/capabilities.h               |  5 ++
>  arch/x86/kvm/vmx/pmu_intel.c                  | 68 +++++++++++++++++++
>  arch/x86/kvm/vmx/vmx.c                        | 54 +++++++++++++++
>  arch/x86/kvm/vmx/vmx.h                        |  1 +
>  arch/x86/kvm/x86.c                            |  2 +-
>  8 files changed, 163 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b5493a7f8f22..e0ff16746a08 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3284,6 +3284,36 @@ Kernel parameters
>  			[KVM,Intel] Control nested virtualization feature in
>  			KVM/VMX. Default is 1 (enabled).
>  
> +	kvm-intel.perfmon_mask=
> +			[KVM,Intel] Defines the host-wide PMU resource
> +			partition between guest and host. Resources assigned
> +			to the guest are unavailable to the host, and vice
> +			versa, while a guest is running.
> +
> +			The value is a semicolon-separated list of terms:
> +
> +			  guest_gp=<list>    General purpose counters
> +					     (0-based) assigned to the guest.
> +			  guest_fixed=<list> Fixed counters (0-based) assigned
> +					     to the guest.
> +			  perf_metrics       PERF_METRICS is assigned to the
> +					     guest. Requires fixed counter 3
> +					     to also be assigned to the
> +					     guest.
> +
> +			<list> is a comma-separated list of numbers and/or
> +			ranges, e.g. "0-2,5". Terms and the resources they
> +			don't mention default to being host-owned. For
> +			example:
> +
> +				kvm-intel.perfmon_mask=guest_gp=0-3;guest_fixed=0-1,3;perf_metrics
> +
> +			assigns general purpose counters 0-3, fixed counters
> +			0, 1 and 3, and PERF_METRICS to the guest, while all
> +			other resources remain with the host.
> +
> +			Default is "" (disabled).
> +
>  	kvm-intel.unrestricted_guest=
>  			[KVM,Intel] Control KVM's use of unrestricted guest
>  			feature (virtualized real and unpaged mode). Default
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index c022337d0bec..92ff685d11b3 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -43,6 +43,9 @@ module_param(enable_pmu, bool, 0444);
>  bool __read_mostly enable_mediated_pmu;
>  EXPORT_SYMBOL_FOR_KVM_INTERNAL(enable_mediated_pmu);
>  
> +u64 __read_mostly perfmon_mask;
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(perfmon_mask);
> +
>  struct kvm_x86_pmu_event_filter {
>  	__u32 action;
>  	__u32 nevents;
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 8322bbed2d64..2dc12e3f3af0 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -64,6 +64,7 @@ struct kvm_pmu_ops {
>  
>  extern bool enable_pmu;
>  extern bool enable_mediated_pmu;
> +extern u64 perfmon_mask;
>  
>  void kvm_pmu_ops_update(const struct kvm_pmu_ops *pmu_ops);
>  
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index 810119167f79..d4c362093966 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -296,6 +296,11 @@ static inline bool cpu_has_vmx_ipiv(void)
>  	return vmcs_config.cpu_based_3rd_exec_ctrl & TERTIARY_EXEC_IPI_VIRT;
>  }
>  
> +static inline bool cpu_has_vmx_perfmon_mask(void)
> +{
> +	return false;
> +}
> +
>  static inline bool cpu_has_vmx_flexpriority(void)
>  {
>  	return cpu_has_vmx_tpr_shadow() &&
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index e2e51006ca47..62e542eac05e 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -955,6 +955,74 @@ static void intel_mediated_pmu_put(struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> +static bool intel_pmu_validate_perfmon_mask(void)
> +{
> +	u64 guest_fixed_mask, guest_gp_mask;
> +
> +	/*
> +	 * Combining VM-exit MSR-store with PerfMon masking produces
> +	 * undefined behavior, so it requires hardware support for this
> +	 * dedicated save control.
> +	 */
> +	if (!cpu_has_save_perf_global_ctrl())
> +		return false;
> +
> +	guest_gp_mask = perfmon_mask & GENMASK_ULL(INTEL_PMC_MAX_GENERIC - 1, 0);
> +	guest_fixed_mask = perfmon_mask >> INTEL_PMC_IDX_FIXED;
> +	guest_fixed_mask &= GENMASK_ULL(INTEL_PMC_MAX_FIXED - 1, 0);
> +
> +	if ((guest_fixed_mask & ~kvm_pmu_cap.fixed_cntr_mask64) ||
> +	    (guest_gp_mask & ~kvm_pmu_cap.cntr_mask64))
> +		return false;
> +
> +	/*
> +	 * Without KVM Arch PerfMon extension support, the guest cannot own
> +	 * non-contiguous GP counters.
> +	 */
> +	if (guest_gp_mask & (guest_gp_mask + 1))
> +		return false;
> +
> +	if ((perfmon_mask & GLOBAL_STATUS_PERF_METRICS_OVF) &&
> +	    !(kvm_host.perf_capabilities & PERF_CAP_PERF_METRICS))
> +		return false;
> +
> +	/*
> +	 * PERF_METRICS and fixed counter 3 must both be host-owned or both
> +	 * guest-owned.
> +	 */
> +	if (!!(perfmon_mask & BIT_ULL(GLOBAL_STATUS_PERF_METRICS_OVF_BIT)) !=
> +	    !!(perfmon_mask & BIT_ULL(INTEL_PMC_IDX_FIXED + 3)))
> +		return false;
> +
> +	/*
> +	 * The guest must not own all PMU counters. Otherwise, the configuration
> +	 * degenerates into plain mediated vPMU and adds unnecessary complexity
> +	 * to the perf scheduler.
> +	 */
> +	if ((guest_fixed_mask == kvm_host_pmu.fixed_cntr_mask64) &&
> +	    (guest_gp_mask == kvm_host_pmu.cntr_mask64))
> +		return false;
> +
> +	return true;
> +}
> +
> +void intel_pmu_perfmon_mask_setup(void)
> +{
> +	if (!perfmon_mask)
> +		return;
> +
> +	if (!enable_mediated_pmu || !cpu_has_vmx_perfmon_mask()) {
> +		perfmon_mask = 0;
> +		return;
> +	}
> +
> +	if (!intel_pmu_validate_perfmon_mask()) {
> +		pr_warn("Invalid perfmon_mask=%#llx, disabling PerfMon masking\n",
> +			perfmon_mask);
> +		perfmon_mask = 0;
> +	}
> +}
> +
>  struct kvm_pmu_ops intel_pmu_ops __initdata = {
>  	.emulate_rdpmc = intel_emulate_rdpmc,
>  	.msr_idx_to_pmc = intel_msr_idx_to_pmc,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ded63e2e39ce..cdd141d22efa 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -14,6 +14,7 @@
>   */
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>  
> +#include <linux/bitmap.h>
>  #include <linux/highmem.h>
>  #include <linux/hrtimer.h>
>  #include <linux/kernel.h>
> @@ -25,6 +26,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/smt.h>
>  #include <linux/slab.h>
> +#include <linux/string.h>
>  #include <linux/tboot.h>
>  #include <linux/trace_events.h>
>  
> @@ -164,6 +166,56 @@ module_param(allow_smaller_maxphyaddr, bool, S_IRUGO);
>  
>  module_param(enable_mediated_pmu, bool, 0444);
>  
> +/*
> + * See the "kvm-intel.perfmon_mask" entry in
> + * Documentation/admin-guide/kernel-parameters.txt for the full syntax.
> + * Example: kvm-intel.perfmon_mask=guest_gp=0-3;guest_fixed=0-1,3;perf_metrics
> + */
> +static int perfmon_mask_set(const char *val, const struct kernel_param *kp)
> +{
> +	unsigned long gp_bitmap = 0, fixed_bitmap = 0;
> +	char *buf, *orig, *tok;
> +	size_t prefix_len;
> +	u64 mask = 0;
> +	int r = 0;
> +
> +	buf = orig = kstrdup(val, GFP_KERNEL);
> +	if (!buf)
> +		return -ENOMEM;
> +
> +	while ((tok = strsep(&buf, ";")) != NULL) {
> +		if (!*tok)
> +			continue;
> +
> +		if (!strcmp(tok, "perf_metrics")) {
> +			mask |= GLOBAL_STATUS_PERF_METRICS_OVF;
> +		} else if ((prefix_len = str_has_prefix(tok, "guest_gp="))) {
> +			r = bitmap_parselist(tok + prefix_len, &gp_bitmap,
> +					     INTEL_PMC_MAX_GENERIC);
> +		} else if ((prefix_len = str_has_prefix(tok, "guest_fixed="))) {
> +			r = bitmap_parselist(tok + prefix_len, &fixed_bitmap,
> +					     INTEL_PMC_MAX_FIXED);
> +		} else {
> +			r = -EINVAL;
> +		}
> +
> +		if (r)
> +			goto out;
> +	}
> +
> +	mask |= gp_bitmap | ((u64)fixed_bitmap << INTEL_PMC_IDX_FIXED);
> +	*(u64 *)kp->arg = mask;
> +out:
> +	kfree(orig);
> +	return r;
> +}
> +
> +static const struct kernel_param_ops perfmon_mask_ops = {
> +	.set = perfmon_mask_set,
> +	.get = param_get_ullong,
> +};
> +module_param_cb(perfmon_mask, &perfmon_mask_ops, &perfmon_mask, 0444);
> +
>  #define KVM_VM_CR0_ALWAYS_OFF (X86_CR0_NW | X86_CR0_CD)
>  #define KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST X86_CR0_NE
>  #define KVM_VM_CR0_ALWAYS_ON				\
> @@ -8824,6 +8876,8 @@ __init int vmx_hardware_setup(void)
>  	else
>  		vt_init_ops.handle_intel_pt_intr = NULL;
>  
> +	intel_pmu_perfmon_mask_setup();
> +
>  	setup_default_sgx_lepubkeyhash();
>  
>  	vmx_set_cpu_caps();
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index dc8517f15bc4..ccda5c5c8c2c 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -667,6 +667,7 @@ static __always_inline struct vcpu_vmx *to_vmx(struct kvm_vcpu *vcpu)
>  void intel_pmu_cross_mapped_check(struct kvm_pmu *pmu);
>  int intel_pmu_create_guest_lbr_event(struct kvm_vcpu *vcpu);
>  void vmx_passthrough_lbr_msrs(struct kvm_vcpu *vcpu);
> +void intel_pmu_perfmon_mask_setup(void);
>  
>  struct vmcs *alloc_vmcs_cpu(bool shadow, int cpu, gfp_t flags);
>  void free_vmcs(struct vmcs *vmcs);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5f3215915c76..26a3b7a267b5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9328,7 +9328,7 @@ int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
>  	if (enable_mediated_pmu && kvm->arch.enable_pmu &&
>  	    !kvm->arch.created_mediated_pmu) {
>  		if (irqchip_in_kernel(kvm)) {
> -			r = perf_create_mediated_pmu(0);
> +			r = perf_create_mediated_pmu(perfmon_mask);
>  			if (r) {
>  				pr_warn_ratelimited(PERF_MEDIATED_PMU_MSG);
>  				return r;