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KVM: x86/pmu: Expose enable_mediated_pmu parameter to user space

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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>
This commit is contained in:
Dapeng Mi 2025-12-05 16:17:09 -08:00 committed by Sean Christopherson
parent 3b36160d94
commit 860bcb1021
3 changed files with 53 additions and 0 deletions
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49
Documentation/admin-guide/kernel-parameters.txt
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@ -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).

2
arch/x86/kvm/svm/svm.c
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@ -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";

2
arch/x86/kvm/vmx/vmx.c
View File

@ -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 \