[PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

Raghavendra Rao Ananta
2023-08-17T00:31:32Z

From: Reiji Watanabe <reijiw@google.com>

KVM does not yet support userspace modifying PMCR_EL0.N (With
the previous patch, KVM ignores what is written by upserspace).
Add support userspace limiting PMCR_EL0.N.

Disallow userspace to set PMCR_EL0.N to a value that is greater
than the host value (KVM_SET_ONE_REG will fail), as KVM doesn't
support more event counters than the host HW implements.
Although this is an ABI change, this change only affects
userspace setting PMCR_EL0.N to a larger value than the host.
As accesses to unadvertised event counters indices is CONSTRAINED
UNPREDICTABLE behavior, and PMCR_EL0.N was reset to the host value
on every vCPU reset before this series, I can't think of any
use case where a user space would do that.

Also, ignore writes to read-only bits that are cleared on vCPU reset,
and RES{0,1} bits (including writable bits that KVM doesn't support
yet), as those bits shouldn't be modified (at least with
the current KVM).

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
---
arch/arm64/include/asm/kvm_host.h | 3 ++
arch/arm64/kvm/pmu-emul.c | 1 +
arch/arm64/kvm/sys_regs.c | 49 +++++++++++++++++++++++++++++--
3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 0f2dbbe8f6a7e..c15ec365283d1 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -259,6 +259,9 @@ struct kvm_arch {
/* PMCR_EL0.N value for the guest */
u8 pmcr_n;

+ /* Limit value of PMCR_EL0.N for the guest */
+ u8 pmcr_n_limit;
+
/* Hypercall features firmware registers' descriptor */
struct kvm_smccc_features smccc_feat;
struct maple_tree smccc_filter;
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index ce7de6bbdc967..39ad56a71ad20 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
* while the latter does not.
*/
kvm->arch.pmcr_n = arm_pmu->num_events - 1;
+ kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;

return 0;
}
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 2075901356c5b..c01d62afa7db4 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
return 0;
}

+static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
+ u64 val)
+{
+ struct kvm *kvm = vcpu->kvm;
+ u64 new_n, mutable_mask;
+ int ret = 0;
+
+ new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
+
+ mutex_lock(&kvm->arch.config_lock);
+ if (unlikely(new_n != kvm->arch.pmcr_n)) {
+ /*
+ * The vCPU can't have more counters than the PMU
+ * hardware implements.
+ */
+ if (new_n <= kvm->arch.pmcr_n_limit)
+ kvm->arch.pmcr_n = new_n;
+ else
+ ret = -EINVAL;
+ }
+ mutex_unlock(&kvm->arch.config_lock);
+ if (ret)
+ return ret;
+
+ /*
+ * Ignore writes to RES0 bits, read only bits that are cleared on
+ * vCPU reset, and writable bits that KVM doesn't support yet.
+ * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
+ * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the vCPU.
+ * But, we leave the bit as it is here, as the vCPU's PMUver might
+ * be changed later (NOTE: the bit will be cleared on first vCPU run
+ * if necessary).
+ */
+ mutable_mask = (ARMV8_PMU_PMCR_MASK | ARMV8_PMU_PMCR_N);
+ val &= mutable_mask;
+ val |= (__vcpu_sys_reg(vcpu, r->reg) & ~mutable_mask);
+
+ /* The LC bit is RES1 when AArch32 is not supported */
+ if (!kvm_supports_32bit_el0())
+ val |= ARMV8_PMU_PMCR_LC;
+
+ __vcpu_sys_reg(vcpu, r->reg) = val;
+ return 0;
+}
+
/* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
#define DBG_BCR_BVR_WCR_WVR_EL1(n) \
{ SYS_DESC(SYS_DBGBVRn_EL1(n)), \
@@ -2147,8 +2192,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
{ SYS_DESC(SYS_CTR_EL0), access_ctr },
{ SYS_DESC(SYS_SVCR), undef_access },

- { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr,
- .reset = reset_pmcr, .reg = PMCR_EL0, .get_user = get_pmcr },
+ { PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
+ .reg = PMCR_EL0, .get_user = get_pmcr, .set_user = set_pmcr },
{ PMU_SYS_REG(PMCNTENSET_EL0),
.access = access_pmcnten, .reg = PMCNTENSET_EL0 },
{ PMU_SYS_REG(PMCNTENCLR_EL0),
--
2.41.0.694.ge786442a9b-goog