Revert "KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible"

Due to a likely mismerge of patches, KVM ended up with a superfluous
commit to "enable" AVIC's fast path for x2AVIC mode.  Even worse, the
superfluous commit has several bugs and creates a nasty local shadow
variable.

Rather than fix the bugs piece-by-piece[*] to achieve the same end
result, revert the patch wholesale.

Opportunistically add a comment documenting the x2AVIC dependencies.

This reverts commit 8c9e639da435874fb845c4d296ce55664071ea7a.

[*] https://lore.kernel.org/all/YxEP7ZBRIuFWhnYJ@google.com

Fixes: 8c9e639da435 ("KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible")
Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230106011306.85230-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 0f67fd3..e4b5f8b 100644 (file)
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -378,7 +378,17 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 
                logid_index = cluster + __ffs(bitmap);
 
-               if (!apic_x2apic_mode(source)) {
+               if (apic_x2apic_mode(source)) {
+                       /*
+                        * 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.
+                        */
+                       l1_physical_id = logid_index;
+               } else {
                        u32 *avic_logical_id_table =
                                page_address(kvm_svm->avic_logical_id_table_page);
 
@@ -393,23 +403,6 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 
                        l1_physical_id = logid_entry &
                                         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.
-                        */
-                       int cluster = (icrh & 0xffff0000) >> 16;
-                       int apic = ffs(icrh & 0xffff) - 1;
-
-                       /*
-                        * If the x2APIC logical ID sub-field (i.e. icrh[15:0])
-                        * contains anything but a single bit, we cannot use the
-                        * fast path, because it is limited to a single vCPU.
-                        */
-                       if (apic < 0 || icrh != (1 << apic))
-                               return -EINVAL;
-
-                       l1_physical_id = (cluster << 4) + apic;
                }
        }