diff mbox series

[v6,02/11] KVM: arm64: PMU: Set the default PMU for the guest on vCPU reset

Message ID 20230926234008.2348607-3-rananta@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: arm64: PMU: Allow userspace to limit the number of PMCs on vCPU | expand

Commit Message

Raghavendra Rao Ananta Sept. 26, 2023, 11:39 p.m. UTC 
From: Reiji Watanabe <reijiw@google.com>

The following patches will use the number of counters information
from the arm_pmu and use this to set the PMCR.N for the guest
during vCPU reset. However, since the guest is not associated
with any arm_pmu until userspace configures the vPMU device
attributes, and a reset can happen before this event, call
kvm_arm_support_pmu_v3() just before doing the reset.

No functional change intended.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
---
 arch/arm64/kvm/pmu-emul.c | 12 ++----------
 arch/arm64/kvm/reset.c    | 18 +++++++++++++-----
 include/kvm/arm_pmu.h     |  6 ++++++
 3 files changed, 21 insertions(+), 15 deletions(-)

Comments

Oliver Upton Sept. 27, 2023, 8:02 a.m. UTC | #1 
Hi Raghu,

On Tue, Sep 26, 2023 at 11:39:59PM +0000, Raghavendra Rao Ananta wrote:
> From: Reiji Watanabe <reijiw@google.com>
> 
> The following patches will use the number of counters information
> from the arm_pmu and use this to set the PMCR.N for the guest
> during vCPU reset. However, since the guest is not associated
> with any arm_pmu until userspace configures the vPMU device
> attributes, and a reset can happen before this event, call
> kvm_arm_support_pmu_v3() just before doing the reset.
> 
> No functional change intended.

I would argue there still is a functional change here, as PMU
initialization failure now shows up on a completely different ioctl for
userspace.

> @@ -216,6 +217,18 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  	vcpu->arch.reset_state.reset = false;
>  	spin_unlock(&vcpu->arch.mp_state_lock);
>  
> +	if (kvm_vcpu_has_pmu(vcpu)) {
> +		if (!kvm_arm_support_pmu_v3())
> +			return -EINVAL;
> +
> +		/*
> +		 * When the vCPU has a PMU, but no PMU is set for the guest
> +		 * yet, set the default one.
> +		 */
> +		if (unlikely(!kvm->arch.arm_pmu) && kvm_arm_set_default_pmu(kvm))
> +			return -EINVAL;
> +	}
> +

Ah, this probably will not mix well with my recent change to get rid of
the return value altogether from kvm_reset_vcpu() [*]. I see two ways to
handle this:

 - Add a separate helper responsible for one-time setup of the vCPU
   called from KVM_ARM_VCPU_INIT which may fail.

 - Add a check for !kvm->arch.arm_pmu to kvm_arm_pmu_v3_init().

No strong preference, though.

[*]: https://lore.kernel.org/r/20230920195036.1169791-8-oliver.upton@linux.dev
Raghavendra Rao Ananta Sept. 27, 2023, 5:36 p.m. UTC | #2 
Hi Oliver,

On Wed, Sep 27, 2023 at 1:02 AM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> Hi Raghu,
>
> On Tue, Sep 26, 2023 at 11:39:59PM +0000, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@google.com>
> >
> > The following patches will use the number of counters information
> > from the arm_pmu and use this to set the PMCR.N for the guest
> > during vCPU reset. However, since the guest is not associated
> > with any arm_pmu until userspace configures the vPMU device
> > attributes, and a reset can happen before this event, call
> > kvm_arm_support_pmu_v3() just before doing the reset.
> >
> > No functional change intended.
>
> I would argue there still is a functional change here, as PMU
> initialization failure now shows up on a completely different ioctl for
> userspace.
>
> > @@ -216,6 +217,18 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> >       vcpu->arch.reset_state.reset = false;
> >       spin_unlock(&vcpu->arch.mp_state_lock);
> >
> > +     if (kvm_vcpu_has_pmu(vcpu)) {
> > +             if (!kvm_arm_support_pmu_v3())
> > +                     return -EINVAL;
> > +
> > +             /*
> > +              * When the vCPU has a PMU, but no PMU is set for the guest
> > +              * yet, set the default one.
> > +              */
> > +             if (unlikely(!kvm->arch.arm_pmu) && kvm_arm_set_default_pmu(kvm))
> > +                     return -EINVAL;
> > +     }
> > +
>
> Ah, this probably will not mix well with my recent change to get rid of
> the return value altogether from kvm_reset_vcpu() [*]. I see two ways to
> handle this:
>
>  - Add a separate helper responsible for one-time setup of the vCPU
>    called from KVM_ARM_VCPU_INIT which may fail.
>
>  - Add a check for !kvm->arch.arm_pmu to kvm_arm_pmu_v3_init().
>
> No strong preference, though.
>
Thanks for the pointer. I think adding it in kvm_arm_pmu_v3_init() may
not be feasible as the reset (reset_pmcr()) may happen before this
init and we may end up setting 0 as PMCR.N for the guest. I'll explore
the other option though.

Thank you.
Raghavendra
> [*]: https://lore.kernel.org/r/20230920195036.1169791-8-oliver.upton@linux.dev
>
> --
> Thanks,
> Oliver
diff mbox series

Patch

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index fb9817bdfeb57..998e1bbd5310d 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -717,8 +717,7 @@  static struct arm_pmu *kvm_pmu_probe_armpmu(void)
 	 * It is still necessary to get a valid cpu, though, to probe for the
 	 * default PMU instance as userspace is not required to specify a PMU
 	 * type. In order to uphold the preexisting behavior KVM selects the
-	 * PMU instance for the core where the first call to the
-	 * KVM_ARM_VCPU_PMU_V3_CTRL attribute group occurs. A dependent use case
+	 * PMU instance for the core during the vcpu reset. A dependent use case
 	 * would be a user with disdain of all things big.LITTLE that affines
 	 * the VMM to a particular cluster of cores.
 	 *
@@ -893,7 +892,7 @@  static void kvm_arm_set_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
  * where vCPUs can be scheduled on any core but the guest
  * counters could stop working.
  */
-static int kvm_arm_set_default_pmu(struct kvm *kvm)
+int kvm_arm_set_default_pmu(struct kvm *kvm)
 {
 	struct arm_pmu *arm_pmu = kvm_pmu_probe_armpmu();
 
@@ -946,13 +945,6 @@  int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
 	if (vcpu->arch.pmu.created)
 		return -EBUSY;
 
-	if (!kvm->arch.arm_pmu) {
-		int ret = kvm_arm_set_default_pmu(kvm);
-
-		if (ret)
-			return ret;
-	}
-
 	switch (attr->attr) {
 	case KVM_ARM_VCPU_PMU_V3_IRQ: {
 		int __user *uaddr = (int __user *)(long)attr->addr;
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 7a65a35ee4ac4..6912832b44b6d 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -206,6 +206,7 @@  static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
  */
 int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 {
+	struct kvm *kvm = vcpu->kvm;
 	struct vcpu_reset_state reset_state;
 	int ret;
 	bool loaded;
@@ -216,6 +217,18 @@  int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	vcpu->arch.reset_state.reset = false;
 	spin_unlock(&vcpu->arch.mp_state_lock);
 
+	if (kvm_vcpu_has_pmu(vcpu)) {
+		if (!kvm_arm_support_pmu_v3())
+			return -EINVAL;
+
+		/*
+		 * When the vCPU has a PMU, but no PMU is set for the guest
+		 * yet, set the default one.
+		 */
+		if (unlikely(!kvm->arch.arm_pmu) && kvm_arm_set_default_pmu(kvm))
+			return -EINVAL;
+	}
+
 	/* Reset PMU outside of the non-preemptible section */
 	kvm_pmu_vcpu_reset(vcpu);
 
@@ -255,11 +268,6 @@  int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	else
 		pstate = VCPU_RESET_PSTATE_EL1;
 
-	if (kvm_vcpu_has_pmu(vcpu) && !kvm_arm_support_pmu_v3()) {
-		ret = -EINVAL;
-		goto out;
-	}
-
 	/* Reset core registers */
 	memset(vcpu_gp_regs(vcpu), 0, sizeof(*vcpu_gp_regs(vcpu)));
 	memset(&vcpu->arch.ctxt.fp_regs, 0, sizeof(vcpu->arch.ctxt.fp_regs));
diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
index 31029f4f7be85..b80c75d80886b 100644
--- a/include/kvm/arm_pmu.h
+++ b/include/kvm/arm_pmu.h
@@ -101,6 +101,7 @@  void kvm_vcpu_pmu_resync_el0(void);
 })
 
 u8 kvm_arm_pmu_get_pmuver_limit(void);
+int kvm_arm_set_default_pmu(struct kvm *kvm);
 
 #else
 struct kvm_pmu {
@@ -174,6 +175,11 @@  static inline u8 kvm_arm_pmu_get_pmuver_limit(void)
 }
 static inline void kvm_vcpu_pmu_resync_el0(void) {}
 
+static inline int kvm_arm_set_default_pmu(struct kvm *kvm)
+{
+	return -ENODEV;
+}
+
 #endif
 
 #endif