LAPIC after reset is in xAPIC mode, which poses a problem for hotplug of
VCPUs with high APIC ID, because reset VCPU is waiting for INIT/SIPI,
but there is no way to uniquely address it using xAPIC.
>From many possible options, we chose the one that also works on real
hardware: accepting interrupts addressed to LAPIC's x2APIC ID even in
xAPIC mode.
KVM intentionally differs from real hardware, because real hardware
(Knights Landing) does just "x2apic_id & 0xff" to decide whether to
accept the interrupt in xAPIC mode and it can deliver one interrupt to
more than one physical destination, e.g. 0x123 to 0x123 and 0x23.
Add a capability to let userspace know that we do something now.
Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
---
arch/x86/kvm/lapic.c | 13 +++++++++++--
arch/x86/kvm/x86.c | 1 +
include/uapi/linux/kvm.h | 1 +
3 files changed, 13 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 42edf1ea2909..b85985985ac8 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -199,10 +199,15 @@ static void recalculate_apic_map(struct kvm *kvm)
xapic_id = kvm_xapic_id(apic);
x2apic_id = kvm_x2apic_id(apic);
- if (apic_x2apic_mode(apic) &&
+ /* Hotplug hack: see kvm_apic_match_physical_addr(), ... */
+ if ((apic_x2apic_mode(apic) || x2apic_id > 0xff) &&
x2apic_id <= new->max_apic_id)
new->phys_map[x2apic_id] = apic;
- else if (!apic_x2apic_mode(apic))
+ /*
+ * ... xAPIC ID of VCPUs with APIC ID > 0xff will wrap-around,
+ * prevent them from masking VCPUs with APIC ID <= 0xff.
+ */
+ if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
new->phys_map[xapic_id] = apic;
ldr = kvm_lapic_get_reg(apic, APIC_LDR);
@@ -612,6 +617,10 @@ static bool kvm_apic_match_physical_addr(struct kvm_lapic *apic, u32 mda)
if (apic_x2apic_mode(apic))
return mda == kvm_x2apic_id(apic);
+ /* Hotplug hack: LAPIC in xAPIC mode also accepts x2APIC. */
+ if (kvm_x2apic_id(apic) > 0xff && mda == kvm_x2apic_id(apic))
+ return true;
+
return mda == kvm_xapic_id(apic);
}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7770d77c828d..945e8eeb4eb1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2618,6 +2618,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_DISABLE_QUIRKS:
case KVM_CAP_SET_BOOT_CPU_ID:
case KVM_CAP_SPLIT_IRQCHIP:
+ case KVM_CAP_X86_ALWAYS_ACCEPT_X2APIC_DEST:
#ifdef CONFIG_KVM_DEVICE_ASSIGNMENT
case KVM_CAP_ASSIGN_DEV_IRQ:
case KVM_CAP_PCI_2_3:
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index e9f5ceffd741..f25efa375255 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -871,6 +871,7 @@ struct kvm_ppc_smmu_info {
#define KVM_CAP_S390_USER_INSTR0 130
#define KVM_CAP_MSI_DEVID 131
#define KVM_CAP_PPC_HTM 132
+#define KVM_CAP_X86_ALWAYS_ACCEPT_X2APIC_DEST 133
#ifdef KVM_CAP_IRQ_ROUTING
--
2.10.2
--
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