linux-kernel - [PATCH v5 4/5] KVM: arm64: PMU: Reload when user modifies registers

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Message-Id: <20250315-pmc-v5-4-ecee87dab216@daynix.com>
Date: Sat, 15 Mar 2025 18:12:13 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
 Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
 Zenghui Yu <yuzenghui@...wei.com>, 
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
 Andrew Jones <andrew.jones@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
 linux-kernel@...r.kernel.org, devel@...nix.com, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH v5 4/5] KVM: arm64: PMU: Reload when user modifies
 registers

Commit d0c94c49792c ("KVM: arm64: Restore PMU configuration on first
run") added the code to reload the PMU configuration on first run.

It is also important to keep the correct state even if system registers
are modified after first run, specifically when debugging Windows on
QEMU with GDB; QEMU tries to write back all visible registers when
resuming the VM execution with GDB, corrupting the PMU state. Windows
always uses the PMU so this can cause adverse effects on that particular
OS.

The usual register writes and reset are already handled independently,
but register writes from userspace are not covered.
Trigger the code to reload the PMU configuration for them instead so
that PMU configuration changes made by users will be applied also after
the first run.

Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
---
 arch/arm64/kvm/pmu-emul.c | 3 ---
 arch/arm64/kvm/sys_regs.c | 4 ++++
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 593216bc14f0..8e10124a7420 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -917,9 +917,6 @@ int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
 		   return -EINVAL;
 	}
 
-	/* One-off reload of the PMU on first run */
-	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
-
 	return 0;
 }
 
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 4d1ef47d0049..727579acc7f6 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1070,6 +1070,8 @@ static int set_pmreg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r, u64 va
 	u64 mask = kvm_pmu_accessible_counter_mask(vcpu);
 
 	__vcpu_sys_reg(vcpu, r->reg) = val & mask;
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }
 
@@ -1228,6 +1230,8 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 		val |= ARMV8_PMU_PMCR_LC;
 
 	__vcpu_sys_reg(vcpu, r->reg) = val;
+	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
+
 	return 0;
 }
 

-- 
2.48.1