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Subject[PATCH 4.9 17/38] KVM: lapic: Fixup LDR on load in x2apic
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Dr. David Alan Gilbert <>

commit 12806ba937382fdfdbad62a399aa2dce65c10fcd upstream.

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:

Reported-by: Yiqian Wei <>
Signed-off-by: Dr. David Alan Gilbert <>
[Applied fixup from Liran Alon. - Paolo]
Signed-off-by: Paolo Bonzini <>
Signed-off-by: Greg Kroah-Hartman <>

arch/x86/kvm/lapic.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2034,6 +2034,7 @@ static int kvm_apic_state_fixup(struct k
if (apic_x2apic_mode(vcpu->arch.apic)) {
u32 *id = (u32 *)(s->regs + APIC_ID);
+ u32 *ldr = (u32 *)(s->regs + APIC_LDR);

if (vcpu->kvm->arch.x2apic_format) {
if (*id != vcpu->vcpu_id)
@@ -2044,6 +2045,10 @@ static int kvm_apic_state_fixup(struct k
*id <<= 24;
+ /* In x2APIC mode, the LDR is fixed and based on the id */
+ if (set)
+ *ldr = kvm_apic_calc_x2apic_ldr(*id);

return 0;

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