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 <dapeng1.mi@linux.intel.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-34-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a8d0afde7f85a506b827ae31d48fc5d9dbabc095..c13a8877f5b378f6261e53bc527fc50e4906705c 100644 (file)
--- 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 1a616eb3ff1c8c3f392e310493e864996e2d287d..5910088fe22ab0c5f0d395a1e9026fe04b1c43c0 100644 (file)
--- 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 f0a20ff2a941aadf69908f584505249d8df5f6ad..62ba2a2b9e98633b37490eef07d953d8fc1106f6 100644 (file)
--- 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                           \