[PATCH v2 4/4] KVM: x86: allow hotplug of VCPU with APIC ID over 0xff

["Radim Krčmář" <rkrcmar@redhat.com>]

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.

Fixes: 682f732ecf73 ("KVM: x86: bump MAX_VCPUS to 288")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 v2:
 * Do not add capability [David, Paolo]
 * Tag it as a fix [David]
---
 arch/x86/kvm/lapic.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index bc8df7ce2766..f1b22bb76a4a 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);
 }
 
-- 
2.11.0