[PATCH 3/4] KVM: arm64: vgic-v4: Fall back to software irqbypass if LPI not found

[Oliver Upton <oliver.upton@linux.dev>]

Continuing with the theme of broken VMMs and guests, irqbypass
registration can fail if the virtual ITS lacks a translation for the
MSI. Either the guest hasn't mapped it or userspace may have forgotten
to restore the ITS.

Exit silently and allow irqbypass configuration to succeed. As a reward
for ingenuity, LPIs are demoted to software injection.

Tested-by: Sudheer Dantuluri <dantuluris@google.com>
Fixes: 196b136498b3 ("KVM: arm/arm64: GICv4: Wire mapping/unmapping of VLPIs in VFIO irq bypass")
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic-v4.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
index 3e1de40720e0..9f51d58b5f8a 100644
--- a/arch/arm64/kvm/vgic/vgic-v4.c
+++ b/arch/arm64/kvm/vgic/vgic-v4.c
@@ -415,7 +415,7 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int virq,
 	struct vgic_irq *irq;
 	struct its_vlpi_map map;
 	unsigned long flags;
-	int ret;
+	int ret = 0;
 
 	if (!vgic_supports_direct_msis(kvm))
 		return 0;
@@ -430,10 +430,15 @@ int kvm_vgic_v4_set_forwarding(struct kvm *kvm, int virq,
 
 	mutex_lock(&its->its_lock);
 
-	/* Perform the actual DevID/EventID -> LPI translation. */
-	ret = vgic_its_resolve_lpi(kvm, its, irq_entry->msi.devid,
-				   irq_entry->msi.data, &irq);
-	if (ret)
+	/*
+	 * Perform the actual DevID/EventID -> LPI translation.
+	 *
+	 * Silently exit if translation fails as the guest (or userspace!) has
+	 * managed to do something stupid. Emulated LPI injection will still
+	 * work if the guest figures itself out at a later time.
+	 */
+	if (vgic_its_resolve_lpi(kvm, its, irq_entry->msi.devid,
+				 irq_entry->msi.data, &irq))
 		goto out;
 
 	/* Silently exit if the vLPI is already mapped */
-- 
2.39.5