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Commit 5bd5db38 authored by Radim Krčmář's avatar Radim Krčmář
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KVM: x86: allow hotplug of VCPU with APIC ID over 0xff 



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: 682f732e ("KVM: x86: bump MAX_VCPUS to 288")
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent b4535b58

arch/x86/kvm/lapic.c

+25 −5
Original line number Diff line number Diff line
@@ -189,15 +189,26 @@ static void recalculate_apic_map(struct kvm *kvm) 
		struct kvm_lapic *apic = vcpu->arch.apic;

		struct kvm_lapic **cluster;

		u16 mask;

		u32 ldr, aid;

		u32 ldr;

		u8 xapic_id;

		u32 x2apic_id;



		if (!kvm_apic_present(vcpu))

			continue;



		aid = apic_x2apic_mode(apic) ? kvm_x2apic_id(apic)

		                             : kvm_xapic_id(apic);

		if (aid <= new->max_apic_id)

			new->phys_map[aid] = apic;

		xapic_id = kvm_xapic_id(apic);

		x2apic_id = kvm_x2apic_id(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;

		/*

		 * ... 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);
@@ -604,6 +615,15 @@ 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: Make LAPIC in xAPIC mode also accept interrupts as if

	 * it were in x2APIC mode.  Hotplugged VCPUs start in xAPIC mode and

	 * this allows unique addressing of VCPUs with APIC ID over 0xff.

	 * The 0xff condition is needed because writeable xAPIC ID.

	 */

	if (kvm_x2apic_id(apic) > 0xff && mda == kvm_x2apic_id(apic))

		return true;



	return mda == kvm_xapic_id(apic);

}