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@xxxxxxxxxx>
---
v3:
* move code that populated physical_map from patch 2 [David]
* extend a comment kvm_apic_match_physical_addr()
v2:
* Do not add capability [David, Paolo]
* Tag it as a fix [David]
---
arch/x86/kvm/lapic.c | 30 +++++++++++++++++++++++++-----
1 file changed, 25 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 519b72bbf947..c6758bdc2c24 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -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);
@@ -606,6 +617,15 @@ 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);
}
--
2.11.0
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
