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    Subject[PATCH v3 12/28] Revert "KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible"
    From
    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 e35e9363e7ff..605c36569ddf 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;
    }
    }

    --
    2.37.3.968.ga6b4b080e4-goog
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