From: Dapeng Mi Date: Sat, 6 Dec 2025 00:17:09 +0000 (-0800) Subject: KVM: x86/pmu: Expose enable_mediated_pmu parameter to user space X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=860bcb1021f5234820592853d56ca12f69e9c81f;p=thirdparty%2Flinux.git KVM: x86/pmu: Expose enable_mediated_pmu parameter to user space Expose enable_mediated_pmu parameter to user space, i.e. allow userspace to enable/disable mediated vPMU support. Document the mediated versus perf-based behavior as part of the kernel-parameters.txt entry, and opportunistically add an entry for the core enable_pmu param as well. Signed-off-by: Dapeng Mi Signed-off-by: Mingwei Zhang Tested-by: Xudong Hao Co-developed-by: Sean Christopherson Tested-by: Manali Shukla Link: https://patch.msgid.link/20251206001720.468579-34-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index a8d0afde7f85a..c13a8877f5b37 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -3044,6 +3044,26 @@ Kernel parameters Default is Y (on). + kvm.enable_pmu=[KVM,X86] + If enabled, KVM will virtualize PMU functionality based + on the virtual CPU model defined by userspace. This + can be overridden on a per-VM basis via + KVM_CAP_PMU_CAPABILITY. + + If disabled, KVM will not virtualize PMU functionality, + e.g. MSRs, PMCs, PMIs, etc., even if userspace defines + a virtual CPU model that contains PMU assets. + + Note, KVM's vPMU support implicitly requires running + with an in-kernel local APIC, e.g. to deliver PMIs to + the guest. Running without an in-kernel local APIC is + not supported, though KVM will allow such a combination + (with severely degraded functionality). + + See also enable_mediated_pmu. + + Default is Y (on). + kvm.enable_virt_at_load=[KVM,ARM64,LOONGARCH,MIPS,RISCV,X86] If enabled, KVM will enable virtualization in hardware when KVM is loaded, and disable virtualization when KVM @@ -3090,6 +3110,35 @@ Kernel parameters If the value is 0 (the default), KVM will pick a period based on the ratio, such that a page is zapped after 1 hour on average. + kvm-{amd,intel}.enable_mediated_pmu=[KVM,AMD,INTEL] + If enabled, KVM will provide a mediated virtual PMU, + instead of the default perf-based virtual PMU (if + kvm.enable_pmu is true and PMU is enumerated via the + virtual CPU model). + + With a perf-based vPMU, KVM operates as a user of perf, + i.e. emulates guest PMU counters using perf events. + KVM-created perf events are managed by perf as regular + (guest-only) events, e.g. are scheduled in/out, contend + for hardware resources, etc. Using a perf-based vPMU + allows guest and host usage of the PMU to co-exist, but + incurs non-trivial overhead and can result in silently + dropped guest events (due to resource contention). + + With a mediated vPMU, hardware PMU state is context + switched around the world switch to/from the guest. + KVM mediates which events the guest can utilize, but + gives the guest direct access to all other PMU assets + when possible (KVM may intercept some accesses if the + virtual CPU model provides a subset of hardware PMU + functionality). Using a mediated vPMU significantly + reduces PMU virtualization overhead and eliminates lost + guest events, but is mutually exclusive with using perf + to profile KVM guests and adds latency to most VM-Exits + (to context switch PMU state). + + Default is N (off). + kvm-amd.nested= [KVM,AMD] Control nested virtualization feature in KVM/SVM. Default is 1 (enabled). diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 1a616eb3ff1c8..5910088fe22ab 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -170,6 +170,8 @@ module_param(intercept_smi, bool, 0444); bool vnmi = true; module_param(vnmi, bool, 0444); +module_param(enable_mediated_pmu, bool, 0444); + static bool svm_gp_erratum_intercept = true; static u8 rsm_ins_bytes[] = "\x0f\xaa"; diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index f0a20ff2a941a..62ba2a2b9e986 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -150,6 +150,8 @@ module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO); extern bool __read_mostly allow_smaller_maxphyaddr; module_param(allow_smaller_maxphyaddr, bool, S_IRUGO); +module_param(enable_mediated_pmu, bool, 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 \