[PATCH v6 15/17] KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible

[Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>]

For x2AVIC, the index from incomplete IPI #vmexit info is invalid
for logical cluster mode. Only ICRH/ICRL values can be used
to determine the IPI destination APIC ID.

Since QEMU defines guest physical APIC ID to be the same as
vCPU ID, it can be used to quickly identify the target vCPU to deliver IPI,
and avoid the overhead from searching through all vCPUs to match the target
vCPU.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
---
 arch/x86/kvm/svm/avic.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index bac876bb1cf1..9c439a32c343 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -358,7 +358,26 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 			/* For xAPIC logical mode, the index is for logical APIC table. */
 			apic_id = avic_logical_id_table[index] & 0x1ff;
 		} else {
-			return -EINVAL;
+			/* For x2APIC logical mode, cannot leverage the index.
+			 * Instead, calculate physical ID from logical ID in ICRH.
+			 */
+			int apic;
+			int first = ffs(icrh & 0xffff);
+			int last = fls(icrh & 0xffff);
+			int cluster = (icrh & 0xffff0000) >> 16;
+
+			/*
+			 * If the x2APIC logical ID sub-field (i.e. icrh[15:0]) contains zero
+			 * or more than 1 bits, we cannot match just one vcpu to kick for
+			 * fast path.
+			 */
+			if (!first || (first != last))
+				return -EINVAL;
+
+			apic = first - 1;
+			if ((apic < 0) || (apic > 15) || (cluster >= 0xfffff))
+				return -EINVAL;
+			apic_id = (cluster << 4) + apic;
 		}
 	}
 
-- 
2.25.1