KVM: arm64: Add assignment-specific sysreg accessor

Assigning a value to a system register doesn't do what it is
supposed to be doing if that register is one that has RESx bits.

The main problem is that we use __vcpu_sys_reg(), which can be used
both as a lvalue and rvalue. When used as a lvalue, the bit masking
occurs *before* the new value is assigned, meaning that we (1) do
pointless work on the old cvalue, and (2) potentially assign an
invalid value as we fail to apply the masks to it.

Fix this by providing a new __vcpu_assign_sys_reg() that does
what it says on the tin, and sanitises the *new* value instead of
the old one. This comes with a significant amount of churn.

Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250603070824.1192795-2-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 25c29107f13f..4f0ae8073f78 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -178,7 +178,7 @@ static void kvm_pmu_set_pmc_value(struct kvm_pmc *pmc, u64 val, bool force)
 		val |= lower_32_bits(val);
 	}
 
-	__vcpu_sys_reg(vcpu, reg) = val;
+	__vcpu_assign_sys_reg(vcpu, reg, val);
 
 	/* Recreate the perf event to reflect the updated sample_period */
 	kvm_pmu_create_perf_event(pmc);
@@ -204,7 +204,7 @@ void kvm_pmu_set_counter_value(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
 void kvm_pmu_set_counter_value_user(struct kvm_vcpu *vcpu, u64 select_idx, u64 val)
 {
 	kvm_pmu_release_perf_event(kvm_vcpu_idx_to_pmc(vcpu, select_idx));
-	__vcpu_sys_reg(vcpu, counter_index_to_reg(select_idx)) = val;
+	__vcpu_assign_sys_reg(vcpu, counter_index_to_reg(select_idx), val);
 	kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
 }
 
@@ -239,7 +239,7 @@ static void kvm_pmu_stop_counter(struct kvm_pmc *pmc)
 
 	reg = counter_index_to_reg(pmc->idx);
 
-	__vcpu_sys_reg(vcpu, reg) = val;
+	__vcpu_assign_sys_reg(vcpu, reg, val);
 
 	kvm_pmu_release_perf_event(pmc);
 }
@@ -503,7 +503,7 @@ static void kvm_pmu_counter_increment(struct kvm_vcpu *vcpu,
 		reg = __vcpu_sys_reg(vcpu, counter_index_to_reg(i)) + 1;
 		if (!kvm_pmc_is_64bit(pmc))
 			reg = lower_32_bits(reg);
-		__vcpu_sys_reg(vcpu, counter_index_to_reg(i)) = reg;
+		__vcpu_assign_sys_reg(vcpu, counter_index_to_reg(i), reg);
 
 		/* No overflow? move on */
 		if (kvm_pmc_has_64bit_overflow(pmc) ? reg : lower_32_bits(reg))
@@ -602,7 +602,7 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
 		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
 
 	/* The reset bits don't indicate any state, and shouldn't be saved. */
-	__vcpu_sys_reg(vcpu, PMCR_EL0) = val & ~(ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_P);
+	__vcpu_assign_sys_reg(vcpu, PMCR_EL0, (val & ~(ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_P)));
 
 	if (val & ARMV8_PMU_PMCR_C)
 		kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX, 0);
@@ -779,7 +779,7 @@ void kvm_pmu_set_counter_event_type(struct kvm_vcpu *vcpu, u64 data,
 	u64 reg;
 
 	reg = counter_index_to_evtreg(pmc->idx);
-	__vcpu_sys_reg(vcpu, reg) = data & kvm_pmu_evtyper_mask(vcpu->kvm);
+	__vcpu_assign_sys_reg(vcpu, reg, (data & kvm_pmu_evtyper_mask(vcpu->kvm)));
 
 	kvm_pmu_create_perf_event(pmc);
 }
@@ -1038,7 +1038,7 @@ static void kvm_arm_set_nr_counters(struct kvm *kvm, unsigned int nr)
 			u64 val = __vcpu_sys_reg(vcpu, MDCR_EL2);
 			val &= ~MDCR_EL2_HPMN;
 			val |= FIELD_PREP(MDCR_EL2_HPMN, kvm->arch.nr_pmu_counters);
-			__vcpu_sys_reg(vcpu, MDCR_EL2) = val;
+			__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
 		}
 	}
 }