[PATCH v6 26/44] KVM: x86/pmu: Always stuff GuestOnly=1,HostOnly=0 for mediated PMCs on AMD

Sean Christopherson
2025-12-06T00:18:19Z

From: Sandipan Das <sandipan.das@amd.com>

On AMD platforms, there is no way to restore PerfCntrGlobalCtl at
VM-Entry or clear it at VM-Exit. Since the register states will be
restored before entering and saved after exiting guest context, the
counters can keep ticking and even overflow leading to chaos while
still in host context.

To avoid this, intecept event selectors, which is already done by mediated
PMU. In addition, always set the GuestOnly bit and clear the HostOnly bit
for PMU selectors on AMD. Doing so allows the counters run only in guest
context even if their enable bits are still set after VM exit and before
host/guest PMU context switch.

Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
[sean: massage shortlog]
Tested-by: Xudong Hao <xudong.hao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/pmu.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
index c1ec1962314e..6d5f791126b1 100644
--- a/arch/x86/kvm/svm/pmu.c
+++ b/arch/x86/kvm/svm/pmu.c
@@ -166,7 +166,8 @@ static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
data &= ~pmu->reserved_bits;
if (data != pmc->eventsel) {
pmc->eventsel = data;
- pmc->eventsel_hw = data;
+ pmc->eventsel_hw = (data & ~AMD64_EVENTSEL_HOSTONLY) |
+ AMD64_EVENTSEL_GUESTONLY;
kvm_pmu_request_counter_reprogram(pmc);
}
return 0;
--
2.52.0.223.gf5cc29aaa4-goog