Re: [PATCH v4] KVM: arm64: Add early_param to control WFx trapping

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Oliver Upton <oliver.upton@xxxxxxxxx> writes:

Hi Colton,

On Mon, Apr 22, 2024 at 06:17:16PM +0000, Colton Lewis wrote:
@@ -2653,6 +2653,27 @@
  			[KVM,ARM] Allow use of GICv4 for direct injection of

+	kvm-arm.wfe_trap_policy=
+			[KVM,ARM] Control when to set wfe instruction trap.

nitpick: when referring to the instruction, please capitalize it.

Also, it doesn't hurt to be verbose here and say this cmdline option
"Controls the WFE instruction trap behavior for KVM VMs"

I say this because there is a separate set of trap controls that allow
WFE or WFI to execute in EL0 (i.e. host userspace).

Will do.

+			trap: set wfe instruction trap
+			notrap: clear wfe instruction trap
+			default: set wfe instruction trap only if multiple
+				 tasks are running on the CPU

I would strongly prefer we not make any default behavior user-visible.
The default KVM behavior can (and will) change in the future.

Only the absence of an explicit trap / notrap policy should fall back to
KVM's default heuristics.

Makes sense to me. Will do.

-static inline void vcpu_clear_wfx_traps(struct kvm_vcpu *vcpu)
+static inline void vcpu_clear_wfe_trap(struct kvm_vcpu *vcpu)
  	vcpu->arch.hcr_el2 &= ~HCR_TWE;
+static inline void vcpu_clear_wfi_trap(struct kvm_vcpu *vcpu)
  	if (atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||
  		vcpu->arch.hcr_el2 &= ~HCR_TWI;
@@ -119,12 +123,28 @@ static inline void vcpu_clear_wfx_traps(struct kvm_vcpu *vcpu)
  		vcpu->arch.hcr_el2 |= HCR_TWI;

This helper definitely does not do as it says on the tin. It ignores the
policy requested on the command line and conditionally *sets* TWI. If
the operator believes they know best and ask for a particular trap policy
KVM should uphold it unconditionally. Even if they've managed to shoot
themselves in the foot.

Will do. I was only splitting up what the existing helper did here.

@@ -423,6 +425,12 @@ void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)


+static bool kvm_should_clear_wfx_trap(enum kvm_wfx_trap_policy p)
+	return (p == KVM_WFX_NOTRAP && kvm_vgic_global_state.has_gicv4)
+		|| (p == KVM_WFX_NOTRAP_SINGLE_TASK && single_task_running());

style nitpick: operators should always go on the preceding line for a
multi-line statement.

Will do.

  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
  	struct kvm_s2_mmu *mmu;
@@ -456,10 +464,15 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
  	if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
  		kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);

-	if (single_task_running())
-		vcpu_clear_wfx_traps(vcpu);
+	if (kvm_should_clear_wfx_trap(kvm_wfi_trap_policy))
+		vcpu_clear_wfi_trap(vcpu);
-		vcpu_set_wfx_traps(vcpu);
+		vcpu_set_wfi_trap(vcpu);
+	if (kvm_should_clear_wfx_trap(kvm_wfe_trap_policy))
+		vcpu_clear_wfe_trap(vcpu);
+	else
+		vcpu_set_wfe_trap(vcpu);

  	if (vcpu_has_ptrauth(vcpu))

I find all of the layering rather hard to follow; we don't need
accessors for doing simple bit manipulation.

Rough sketch:

static bool kvm_vcpu_should_clear_twi(struct kvm_vcpu *vcpu)
	if (unlikely(kvm_wfi_trap != KVM_WFX_DEFAULT))
		return kvm_wfi_trap == KVM_WFX_NOTRAP;

	return single_task_running() &&
	       (atomic_read(&vcpu->arch.vgic_cpu.vgic_v3.its_vpe.vlpi_count) ||

static bool kvm_vcpu_should_clear_twe(struct kvm_vcpu *vcpu)
	if (unlikely(kvm_wfe_trap != KVM_WFX_DEFAULT))
		return kvm_wfe_trap == KVM_WFX_NOTRAP;

	return single_task_running();

static void kvm_vcpu_load_compute_hcr(struct kvm_vcpu *vcpu)
	vcpu->arch.hcr_el2 |= HCR_TWE | HCR_TWI;

	if (kvm_vcpu_should_clear_twe(vcpu))
		vcpu->arch.hcr_el2 &= ~HCR_TWE;
	if (kvm_vcpu_should_clear_twi(vcpu))
		vcpu->arch.hcr_el2 &= ~HCR_TWI;

Will do.

And if we really wanted to, the non-default trap configuration could be
moved to vcpu_reset_hcr() if we cared.

Might as well.

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