linux-kernel - [PATCH 09/19] KVM: SVM: Drop duplicate calcuation of AVIC/x2AVIC "logical index"

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Date:   Wed, 31 Aug 2022 00:34:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Li RongQing <lirongqing@...du.com>
Subject: [PATCH 09/19] KVM: SVM: Drop duplicate calcuation of AVIC/x2AVIC
 "logical index"

Drop the duplicate calculation of the logical "index", which ends up
being the same for x2APIC vs. xAPIC: cluster + bit number.

Note, the existing code is a mess and uses ffs(), which is 1-based, and
__ffs(), which is 0-based, for the exact same calculation, i.e. replacing
"ffs(bitmap) - 1" with "__ffs(bitmap)" is intentional.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/avic.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 8c9cad96008e..05a1cde8175c 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -407,12 +407,14 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 					 AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK;
 		} else {
 			/*
-			 * For x2APIC logical mode, cannot leverage the index.
-			 * Instead, calculate physical ID from logical ID in ICRH.
+			 * For x2APIC, the logical APIC ID is a read-only value
+			 * that is derived from the x2APIC ID, thus the x2APIC
+			 * ID can be found by reversing the calculation (done
+			 * above).  Note, bits 31:20 of the x2APIC ID are not
+			 * propagated to the logical ID, but KVM limits the
+			 * x2APIC ID limited to KVM_MAX_VCPU_IDS.
 			 */
-			int apic = ffs(bitmap) - 1;
-
-			l1_physical_id = cluster + apic;
+			l1_physical_id = logid_index;
 		}
 	}
 
-- 
2.37.2.672.g94769d06f0-goog