Re: [PATCH v5 27/33] KVM: SVM: Require logical ID to be power-of-2 for AVIC entry

Maxim Levitsky
2023-01-08T15:22:02Z

On Fri, 2023-01-06 at 01:13 +0000, Sean Christopherson wrote:
> Do not modify AVIC's logical ID table if the logical ID portion of the
> LDR is not a power-of-2, i.e. if the LDR has multiple bits set. Taking
> only the first bit means that KVM will fail to match MDAs that intersect
> with "higher" bits in the "ID"
>
> The "ID" acts as a bitmap, but is referred to as an ID because there's an
> implicit, unenforced "requirement" that software only set one bit. This
> edge case is arguably out-of-spec behavior, but KVM cleanly handles it
> in all other cases, e.g. the optimized logical map (and AVIC!) is also
> disabled in this scenario.
>
> Refactor the code to consolidate the checks, and so that the code looks
> more like avic_kick_target_vcpus_fast().

Very minor nitpick about the subject, it feels like it just changes something
minor but it pretty much rewrites the avic_get_logical_id_entry.

When I bisected for the bug, this did confuse me a bit.

I don't have a good idea on how to call this patch so I won't object to the current
subject to stay as well.

Best regards,
Maxim Levitsky

>
> Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/svm/avic.c | 30 +++++++++++++++---------------
> 1 file changed, 15 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 5b33f91b701c..eb979695e7d8 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -513,26 +513,26 @@ unsigned long avic_vcpu_get_apicv_inhibit_reasons(struct kvm_vcpu *vcpu)
> static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
> {
> struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> - int index;
> u32 *logical_apic_id_table;
> - int dlid = GET_APIC_LOGICAL_ID(ldr);
> + u32 cluster, index;
>
> - if (!dlid)
> - return NULL;
> + ldr = GET_APIC_LOGICAL_ID(ldr);
>
> - if (flat) { /* flat */
> - index = ffs(dlid) - 1;
> - if (index > 7)
> + if (flat) {
> + cluster = 0;
> + } else {
> + cluster = (ldr >> 4);
> + if (cluster >= 0xf)
> return NULL;
> - } else { /* cluster */
> - int cluster = (dlid & 0xf0) >> 4;
> - int apic = ffs(dlid & 0x0f) - 1;
> -
> - if ((apic < 0) || (apic > 7) ||
> - (cluster >= 0xf))
> - return NULL;
> - index = (cluster << 2) + apic;
> + ldr &= 0xf;
> }
> + if (!ldr || !is_power_of_2(ldr))
> + return NULL;
> +
> + index = __ffs(ldr);
> + if (WARN_ON_ONCE(index > 7))
> + return NULL;
> + index += (cluster << 2);
>
> logical_apic_id_table = (u32 *) page_address(kvm_svm->avic_logical_id_table_page);
>

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky