[PATCH v4 12/16] KVM: arm64: nv: Request vPE doorbell upon nested ERET to L2

Marc Zyngier
2025-02-25T17:29:47Z

From: Oliver Upton <oliver.upton@linux.dev>

Running an L2 guest with GICv4 enabled goes absolutely nowhere, and gets
into a vicious cycle of nested ERET followed by nested exception entry
into the L1.

When KVM does a put on a runnable vCPU, it marks the vPE as nonresident
but does not request a doorbell IRQ. Behind the scenes in the ITS
driver's view of the vCPU, its_vpe::pending_last gets set to true to
indicate that context is still runnable.

This comes to a head when doing the nested ERET into L2. The vPE doesn't
get scheduled on the redistributor as it is exclusively part of the L1's
VGIC context. kvm_vgic_vcpu_pending_irq() returns true because the vPE
appears runnable, and KVM does a nested exception entry into the L1
before L2 ever gets off the ground.

This issue can be papered over by requesting a doorbell IRQ when
descheduling a vPE as part of a nested ERET. KVM needs this anyway to
kick the vCPU out of the L2 when an IRQ becomes pending for the L1.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240823212703.3576061-4-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/kvm_host.h | 2 ++
arch/arm64/kvm/emulate-nested.c | 2 ++
arch/arm64/kvm/vgic/vgic-v4.c | 18 +++++++++++++++++-
3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 86519a73971ed..74be23236aaca 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -946,6 +946,8 @@ struct kvm_vcpu_arch {
#define PMUSERENR_ON_CPU __vcpu_single_flag(sflags, BIT(5))
/* WFI instruction trapped */
#define IN_WFI __vcpu_single_flag(sflags, BIT(6))
+/* KVM is currently emulating a nested ERET */
+#define IN_NESTED_ERET __vcpu_single_flag(sflags, BIT(7))

/* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 9986bb88c2597..834c587500699 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2503,6 +2503,7 @@ void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
}

preempt_disable();
+ vcpu_set_flag(vcpu, IN_NESTED_ERET);
kvm_arch_vcpu_put(vcpu);

if (!esr_iss_is_eretax(esr))
@@ -2514,6 +2515,7 @@ void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
*vcpu_cpsr(vcpu) = spsr;

kvm_arch_vcpu_load(vcpu, smp_processor_id());
+ vcpu_clear_flag(vcpu, IN_NESTED_ERET);
preempt_enable();

kvm_pmu_nested_transition(vcpu);
diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
index eedecbbbcf31b..0d9fb235c0180 100644
--- a/arch/arm64/kvm/vgic/vgic-v4.c
+++ b/arch/arm64/kvm/vgic/vgic-v4.c
@@ -336,6 +336,22 @@ void vgic_v4_teardown(struct kvm *kvm)
its_vm->vpes = NULL;
}

+static inline bool vgic_v4_want_doorbell(struct kvm_vcpu *vcpu)
+{
+ if (vcpu_get_flag(vcpu, IN_WFI))
+ return true;
+
+ if (likely(!vcpu_has_nv(vcpu)))
+ return false;
+
+ /*
+ * GICv4 hardware is only ever used for the L1. Mark the vPE (i.e. the
+ * L1 context) nonresident and request a doorbell to kick us out of the
+ * L2 when an IRQ becomes pending.
+ */
+ return vcpu_get_flag(vcpu, IN_NESTED_ERET);
+}
+
int vgic_v4_put(struct kvm_vcpu *vcpu)
{
struct its_vpe *vpe = &vcpu->arch.vgic_cpu.vgic_v3.its_vpe;
@@ -343,7 +359,7 @@ int vgic_v4_put(struct kvm_vcpu *vcpu)
if (!vgic_supports_direct_msis(vcpu->kvm) || !vpe->resident)
return 0;

- return its_make_vpe_non_resident(vpe, !!vcpu_get_flag(vcpu, IN_WFI));
+ return its_make_vpe_non_resident(vpe, vgic_v4_want_doorbell(vcpu));
}

int vgic_v4_load(struct kvm_vcpu *vcpu)
--
2.39.2