[PATCH 2/2] KVM: lapic: Fixup LDR on load in x2apic

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

In x2apic mode the LDR is fixed based on the ID rather
than separately loadable like it was before x2.
When kvm_apic_set_state is called, the base is set, and if
it has the X2APIC_ENABLE flag set then the LDR is calculated;
however that value gets overwritten by the memcpy a few lines
below overwriting it with the value that came from userland.

The symptom is a lack of EOI after loading the state
(e.g. after a QEMU migration) and is due to the EOI bitmap
being wrong due to the incorrect LDR.  This was seen with
a Win2016 guest under Qemu with irqchip=split whose USB mouse
didn't work after a VM migration.

This corresponds to RH bug:
  https://bugzilla.redhat.com/show_bug.cgi?id=1502591

Reported-by: Yiqian Wei <yiwei@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 arch/x86/kvm/lapic.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 4991e9e51611..cff55beb0263 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2201,7 +2201,12 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
 {
 	if (apic_x2apic_mode(vcpu->arch.apic)) {
 		u32 *id = (u32 *)(s->regs + APIC_ID);
+		u32 *ldr = (u32 *)(s->regs + APIC_LDR);
 
+		if (set) {
+			/* In x2 the LDR is fixed based on the id */
+			*ldr = kvm_apic_calc_x2apic_ldr(*id);
+		}
 		if (vcpu->kvm->arch.x2apic_format) {
 			if (*id != vcpu->vcpu_id)
 				return -EINVAL;
-- 
2.14.3