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>

1 files changed, 49 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a8d0afde7f85..c13a8877f5b3 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).