From: Paolo Bonzini <pbonzini@redhat.com>
Date: Mon, 9 Feb 2026 18:35:16 +0000 (+0100)
Subject: Merge tag 'kvm-x86-pmu-6.20' of https://github.com/kvm-x86/linux into HEAD
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bf2c3138ae3694d4687cbe451c774c288ae2ad06;p=thirdparty%2Fkernel%2Fstable.git

Merge tag 'kvm-x86-pmu-6.20' of https://github.com/kvm-x86/linux into HEAD

KVM mediated PMU support for 6.20

Add support for mediated PMUs, where KVM gives the guest full ownership of PMU
hardware (contexted switched around the fastpath run loop) and allows direct
access to data MSRs and PMCs (restricted by the vPMU model), but intercepts
access to control registers, e.g. to enforce event filtering and to prevent the
guest from profiling sensitive host state.

To keep overall complexity reasonable, mediated PMU usage is all or nothing
for a given instance of KVM (controlled via module param).  The Mediated PMU
is disabled default, partly to maintain backwards compatilibity for existing
setup, partly because there are tradeoffs when running with a mediated PMU that
may be non-starters for some use cases, e.g. the host loses the ability to
profile guests with mediated PMUs, the fastpath run loop is also a blind spot,
entry/exit transitions are more expensive, etc.

Versus the emulated PMU, where KVM is "just another perf user", the mediated
PMU delivers more accurate profiling and monitoring (no risk of contention and
thus dropped events), with significantly less overhead (fewer exits and faster
emulation/programming of event selectors) E.g. when running Specint-2017 on
a single-socket Sapphire Rapids with 56 cores and no-SMT, and using perf from
within the guest:

  Perf command:
  a. basic-sampling: perf record -F 1000 -e 6-instructions  -a --overwrite
  b. multiplex-sampling: perf record -F 1000 -e 10-instructions -a --overwrite

  Guest performance overhead:
  ---------------------------------------------------------------------------
  | Test case          | emulated vPMU | all passthrough | passthrough with |
  |                    |               |                 | event filters    |
  ---------------------------------------------------------------------------
  | basic-sampling     |   33.62%      |    4.24%        |   6.21%          |
  ---------------------------------------------------------------------------
  | multiplex-sampling |   79.32%      |    7.34%        |   10.45%         |
  ---------------------------------------------------------------------------
---

bf2c3138ae3694d4687cbe451c774c288ae2ad06
diff --cc arch/x86/kvm/svm/svm.c
index 9ee74c57bd51b,5910088fe22ab..8f8bc863e2143
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@@ -1011,8 -1046,14 +1049,13 @@@ static void svm_recalc_instruction_inte
  		if (vls) {
  			svm_clr_intercept(svm, INTERCEPT_VMLOAD);
  			svm_clr_intercept(svm, INTERCEPT_VMSAVE);
 -			svm->vmcb->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
  		}
  	}
+ 
+ 	if (kvm_need_rdpmc_intercept(vcpu))
+ 		svm_set_intercept(svm, INTERCEPT_RDPMC);
+ 	else
+ 		svm_clr_intercept(svm, INTERCEPT_RDPMC);
  }
  
  static void svm_recalc_intercepts(struct kvm_vcpu *vcpu)
diff --cc arch/x86/kvm/x86.h
index ff20e62d98c63,6e1fb1680c0a3..94d4f07aaaa09
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@@ -481,9 -470,8 +481,10 @@@ extern struct kvm_caps kvm_caps
  extern struct kvm_host_values kvm_host;
  
  extern bool enable_pmu;
+ extern bool enable_mediated_pmu;
  
 +void kvm_setup_xss_caps(void);
 +
  /*
   * Get a filtered version of KVM's supported XCR0 that strips out dynamic
   * features for which the current process doesn't (yet) have permission to use.