Re: [PATCH v11 09/21] KVM: arm64: PMU: Recreate events after MDCR_EL2 changes

On 2026/09/22 15:54, Oliver Upton wrote:

On Sun, Sep 20, 2026 at 08:15:50PM +0900, Akihiko Odaki wrote:

quoted

+void kvm_pmu_apply_mdcr(struct kvm_vcpu *vcpu, u64 old, u64 val)
+{
+	u64 changed = old ^ val;
+
+	/*
+	 * HPMN determines which counters HPMD and HLP apply to. Changes to
+	 * these fields require new perf event filters and sample periods.
+	 */
+	if (changed & (MDCR_EL2_HPMN | MDCR_EL2_HPMD | MDCR_EL2_HLP))
+		kvm_pmu_request_recreate(vcpu);
+	else if (changed & MDCR_EL2_HPME)
+		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+}
+

Urgh... I'm not a fan of this. Can you just modify kvm_vcpu_reload_pmu()
to discard all backing perf events and recompute them only for enabled
PMCs? We set KVM_REQ_RELOAD_PMU pretty rarely.

quoted

  	__vcpu_rmw_sys_reg(vcpu, PMOVSSET_EL0, &=, mask);
  	__vcpu_rmw_sys_reg(vcpu, PMINTENSET_EL1, &=, mask);

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 75624725adf1..7dd2cafce247 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c

@@ -3114,17 +3114,22 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
  	}
  
  	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
-
-	/*
-	 * Request a reload of the PMU to enable/disable the counters
-	 * affected by HPME.
-	 */
-	if ((old ^ val) & MDCR_EL2_HPME)
-		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+	kvm_pmu_apply_mdcr(vcpu, old, val);
  
  	return true;
  }

quoted

+static int set_mdcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
+		    u64 val)
+{
+	u64 old = __vcpu_sys_reg(vcpu, MDCR_EL2);
+
+	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
+	kvm_pmu_apply_mdcr(vcpu, old, val);
+
+	return 0;
+}
+

Just keep the old ^ new test inline for access_mdcr() and set the
request unconditionally from set_mdcr(). In the usual case
KVM_REQ_RELOAD_PMU is already set on a vCPU that userspace is restoring.

Hi Oliver,

Thanks for the feedback. I'll make both changes for v2.

I originally tried to preserve existing behavior and keep things 
consistent, but I agree that simplicity is much better here given how 
rarely KVM_REQ_RELOAD_PMU is set.

Regards,
Akihiko Odaki