Re: [PATCH v3] KVM: x86: Allow APICv APIC ID inhibit to be cleared

[Maxim Levitsky <mlevitsk@redhat.com>]

On Thu, 2022-11-17 at 11:33 -0700, Greg Edwards wrote:
> Legacy kernels prior to commit 4399c03c6780 ("x86/apic: Remove
> verify_local_APIC()") write the APIC ID of the boot CPU twice to verify
> a functioning local APIC.  This results in APIC acceleration inhibited
> on these kernels for reason APICV_INHIBIT_REASON_APIC_ID_MODIFIED.
> 
> Allow the APICV_INHIBIT_REASON_APIC_ID_MODIFIED inhibit reason to be
> cleared if/when all APICs in xAPIC mode set their APIC ID back to the
> expected vcpu_id value.
> 
> Fold the functionality previously in kvm_lapic_xapic_id_updated() into
> kvm_recalculate_apic_map(), as this allows examining all APICs in one
> pass.
> 
> Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
> Signed-off-by: Greg Edwards <gedwards@ddn.com>
> ---
> Changes from v2:
>   * Comment and variable name tweaks.  [Sean]
> 
> Changes from v1:
>   * Fold kvm_lapic_xapic_id_updated() into kvm_recalculate_apic_map() and
>     verify no APICs in xAPIC mode have a modified APIC ID before clearing
>     APICV_INHIBIT_REASON_APIC_ID_MODIFIED.  [Sean]
>   * Rebase on top of Sean's APIC fixes+cleanups series.  [Sean]
>     https://lore.kernel.org/all/20221001005915.2041642-1-seanjc@google.com/
> 
>  arch/x86/kvm/lapic.c | 41 +++++++++++++++--------------------------
>  1 file changed, 15 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9b3af49d2524..5224d73cd84a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -236,6 +236,7 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>         struct kvm_vcpu *vcpu;
>         unsigned long i;
>         u32 max_id = 255; /* enough space for any xAPIC ID */
> +       bool xapic_id_mismatch = false;
>  
>         /* Read kvm->arch.apic_map_dirty before kvm->arch.apic_map.  */
>         if (atomic_read_acquire(&kvm->arch.apic_map_dirty) == CLEAN)
> @@ -285,6 +286,15 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>                 xapic_id = kvm_xapic_id(apic);
>                 x2apic_id = kvm_x2apic_id(apic);
>  
> +               /*
> +                * Deliberately truncate the vCPU ID when detecting a mismatched
> +                * APIC ID to avoid false positives if the vCPU ID, i.e. x2APIC
> +                * ID, is a 32-bit value.  Any unwanted aliasing due to
> +                * truncation results will be detected below.
> +                */
> +               if (!apic_x2apic_mode(apic) && xapic_id != (u8)vcpu->vcpu_id)
> +                       xapic_id_mismatch = true;
> +
>                 /*
>                  * Apply KVM's hotplug hack if userspace has enable 32-bit APIC
>                  * IDs.  Allow sending events to vCPUs by their x2APIC ID even
> @@ -396,6 +406,11 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>         else
>                 kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
>  
> +       if (xapic_id_mismatch)
> +               kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> +       else
> +               kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> +
>         old = rcu_dereference_protected(kvm->arch.apic_map,
>                         lockdep_is_held(&kvm->arch.apic_map_lock));
>         rcu_assign_pointer(kvm->arch.apic_map, new);
> @@ -2155,28 +2170,6 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
>         }
>  }
>  
> -static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
> -{
> -       struct kvm *kvm = apic->vcpu->kvm;
> -
> -       if (!kvm_apic_hw_enabled(apic))
> -               return;
> -
> -       if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
> -               return;
> -
> -       /*
> -        * Deliberately truncate the vCPU ID when detecting a modified APIC ID
> -        * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
> -        * value.  If the wrap/truncation results in unwatned aliasing, APICv
> -        * will be inhibited as part of updating KVM's optimized APIC maps.
> -        */
> -       if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
> -               return;
> -
> -       kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> -}
> -
>  static int get_lvt_index(u32 reg)
>  {
>         if (reg == APIC_LVTCMCI)
> @@ -2197,7 +2190,6 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>         case APIC_ID:           /* Local APIC ID */
>                 if (!apic_x2apic_mode(apic)) {
>                         kvm_apic_set_xapic_id(apic, val >> 24);
> -                       kvm_lapic_xapic_id_updated(apic);
>                 } else {
>                         ret = 1;
>                 }
> @@ -2919,9 +2911,6 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>         }
>         memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
>  
> -       if (!apic_x2apic_mode(vcpu->arch.apic))
> -               kvm_lapic_xapic_id_updated(vcpu->arch.apic);
> -
>         atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
>         kvm_recalculate_apic_map(vcpu->kvm);
>         kvm_apic_set_version(vcpu);


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

Best regards,
	Maxim Levitsky