Commit 860bcb10 authored by Dapeng Mi's avatar Dapeng Mi Committed by Sean Christopherson
Browse files

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: default avatarDapeng Mi <dapeng1.mi@linux.intel.com>
Signed-off-by: default avatarMingwei Zhang <mizhang@google.com>
Tested-by: default avatarXudong Hao <xudong.hao@intel.com>
Co-developed-by: default avatarSean Christopherson <seanjc@google.com>
Tested-by: default avatarManali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-34-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 3b36160d

Documentation/admin-guide/kernel-parameters.txt

+49 −0
Original line number Diff line number Diff line
@@ -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).

arch/x86/kvm/svm/svm.c

+2 −0
Original line number Diff line number Diff line
@@ -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";

arch/x86/kvm/vmx/vmx.c

+2 −0
Original line number Diff line number Diff line
@@ -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				\