[PATCH 2/3] KVM: arm64: vgic-v3: Don't save pending tables with LPIs disabled

[Fuad Tabba <fuad.tabba@linux.dev>]

KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES writes each LPI's pending state
into its redistributor's pending table, clearing the bit of an LPI that
isn't pending. However, once a redistributor has LPIs disabled and RWP
reads 0, "there are no further accesses by the GIC to the LPI Pending
table" (IHI 0069H.b, 5.1.2) until enabling LPIs reads it. A bit the
guest sets there is cleared by the next save, which QEMU runs whenever
it stops the VM, and the guest never takes the LPI.

Skip the LPIs of a redistributor with LPIs disabled, and document it.
On GICv4.1 this drops the state of a forwarded vLPI there, which the
disable is allowed to lose: "There is no guarantee that clearing
GICR_CTLR.EnableLPIs causes the LPI Pending table to be updated in
memory" (5.1.2).

Fixes: 280771252c1ba ("KVM: arm64: vgic-v3: KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 Documentation/virt/kvm/devices/arm-vgic-v3.rst | 1 +
 arch/arm64/kvm/vgic/vgic-v3.c                  | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/devices/arm-vgic-v3.rst b/Documentation/virt/kvm/devices/arm-vgic-v3.rst
index 5395ee66fc324..c878d3f728858 100644
--- a/Documentation/virt/kvm/devices/arm-vgic-v3.rst
+++ b/Documentation/virt/kvm/devices/arm-vgic-v3.rst
@@ -308,6 +308,7 @@ Groups:
       save all LPI pending bits into guest RAM pending tables.
 
       The first kB of the pending table is not altered by this operation.
+      Neither is the pending table of a redistributor with LPIs disabled.
 
   Errors:
 
diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index 65b77fb35be37..9bbf11ddba92e 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -625,7 +625,9 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
 			continue;
 
 		vcpu = irq->target_vcpu;
-		if (!vcpu)
+
+		/* While LPIs are disabled, the pending table is the guest's. */
+		if (!vcpu || !vgic_lpis_enabled(vcpu))
 			goto put_irq;
 
 		pendbase = GICR_PENDBASER_ADDRESS(vcpu->arch.vgic_cpu.pendbaser);
-- 
2.47.3