[PATCH v5 14/33] Revert "KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible"

Sean Christopherson
2023-01-06T01:14:43Z

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>
---
arch/x86/kvm/svm/avic.c | 29 +++++++++++------------------
1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 0f67fd34ef99..e4b5f8b14882 100644
--- 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;
}
}

--
2.39.0.314.g84b9a713c41-goog