Message-ID: <20251206001720.468579-34-seanjc@google.com>
Date: Fri,  5 Dec 2025 16:17:09 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oupton@...nel.org>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Anup Patel <anup@...infault.org>, 
	Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Xin Li <xin@...or.com>, "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, loongarch@...ts.linux.dev, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, Mingwei Zhang <mizhang@...gle.com>, 
	Xudong Hao <xudong.hao@...el.com>, Sandipan Das <sandipan.das@....com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Manali Shukla <manali.shukla@....com>, Jim Mattson <jmattson@...gle.com>
Subject: [PATCH v6 33/44] KVM: x86/pmu: Expose enable_mediated_pmu parameter
 to user space

From: Dapeng Mi <dapeng1.mi@...ux.intel.com>

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@...ux.intel.com>
Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
Tested-by: Xudong Hao <xudong.hao@...el.com>
Co-developed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 .../admin-guide/kernel-parameters.txt         | 49 +++++++++++++++++++
 arch/x86/kvm/svm/svm.c                        |  2 +
 arch/x86/kvm/vmx/vmx.c                        |  2 +
 3 files changed, 53 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6c42061ca20e..ed6f2ed94756 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2926,6 +2926,26 @@
 
 			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
@@ -2972,6 +2992,35 @@
 			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 cbebd3a18918..20fb7c38bf75 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 f0a20ff2a941..62ba2a2b9e98 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				\
-- 
2.52.0.223.gf5cc29aaa4-goog

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