[PATCH v3] KVM: arm64: Add early_param to control WFx trapping

[Colton Lewis]

Add an early_param to control WFx (WFI or WFE) trapping. This is so
interrupts can be passed through if the CPU has support for direct
interrupt injection, a feature of GICv4. This is described as an
enumeration with three possible behaviors, always passthrough (never
trap), never passthrough (always trap), or default (trap if more than
one task is running. Default matches the current behavior.

Signed-off-by: Colton Lewis <coltonlewis@xxxxxxxxxx>
---
v3:
* Changed control mechanism to an early_param on Marc's advice this should be
  a system level decision and not exposed via uapi
* Reduced behavior to an enum from an integer as there are only a few options
  that make logical sense
* Limit option for always passthrough to systems with GICv4 since the primary
  case for always passthrough is systems with direct interrupt injection

v2:
https://lore.kernel.org/kvmarm/20240319164341.1674863-1-coltonlewis@xxxxxxxxxx/

v1:
https://lore.kernel.org/kvmarm/20240129213918.3124494-1-coltonlewis@xxxxxxxxxx/

arch/arm64/include/asm/kvm_host.h |  7 +++++++
 arch/arm64/kvm/arm.c              | 30 +++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 21c57b812569..e9225b1d0e9b 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -67,6 +67,13 @@ enum kvm_mode {
 	KVM_MODE_NV,
 	KVM_MODE_NONE,
 };
+
+enum kvm_interrupt_passthrough {
+	KVM_INTERRUPT_PASSTHROUGH_DEFAULT,
+	KVM_INTERRUPT_PASSTHROUGH_ALWAYS,
+	KVM_INTERRUPT_PASSTHROUGH_NEVER,
+};
+
 #ifdef CONFIG_KVM
 enum kvm_mode kvm_get_mode(void);
 #else
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index a25265aca432..5d0ea6b2c652 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -46,6 +46,7 @@
 #include <kvm/arm_psci.h>

 static enum kvm_mode kvm_mode = KVM_MODE_DEFAULT;
+static enum kvm_interrupt_passthrough kvm_interrupt_passthrough = KVM_INTERRUPT_PASSTHROUGH_DEFAULT;

 DECLARE_KVM_HYP_PER_CPU(unsigned long, kvm_hyp_vector);

@@ -456,7 +457,10 @@ 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())
+	if ((kvm_interrupt_passthrough == KVM_INTERRUPT_PASSTHROUGH_ALWAYS
+	     && kvm_vgic_global_state.has_gicv4) ||
+	    (kvm_interrupt_passthrough == KVM_INTERRUPT_PASSTHROUGH_DEFAULT
+	     && single_task_running()))
 		vcpu_clear_wfx_traps(vcpu);
 	else
 		vcpu_set_wfx_traps(vcpu);
@@ -2654,6 +2658,30 @@ static int __init early_kvm_mode_cfg(char *arg)
 }
 early_param("kvm-arm.mode", early_kvm_mode_cfg);

+static int __init early_kvm_interrupt_passthrough_cfg(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strcmp(arg, "always") == 0) {
+		kvm_interrupt_passthrough = KVM_INTERRUPT_PASSTHROUGH_ALWAYS;
+		return 0;
+	}
+
+	if (strcmp(arg, "never") == 0) {
+		kvm_interrupt_passthrough = KVM_INTERRUPT_PASSTHROUGH_NEVER;
+		return 0;
+	}
+
+	if (strcmp(arg, "default") == 0) {
+		kvm_interrupt_passthrough = KVM_INTERRUPT_PASSTHROUGH_DEFAULT;
+		return 0;
+	}
+
+	return -EINVAL;
+}
+early_param("kvm-arm.interrupt-passthrough", early_kvm_interrupt_passthrough_cfg);
+
 enum kvm_mode kvm_get_mode(void)
 {
 	return kvm_mode;
--
2.44.0.478.gd926399ef9-goog