On Wed, 2022-08-31 at 16:37 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:34 +0000, Sean Christopherson wrote:
> > > Use the already-calculated-and-sanity-checked destination bitmap when
> > > processing a fast AVIC kick in logical mode, and drop the logical path's
> > > flawed logic. The intent of the check is to ensure the bitmap is a power
> > > of two, whereas "icrh != (1 << avic)" effectively checks that the bitmap
> > > is a power of two _and_ the target cluster is '0'.
> > >
> > > Note, the flawed check isn't a functional issue, it simply means that KVM
> > > will go down the slow path if the target cluster is non-zero.
> > >
> > > Fixes: 8c9e639da435 ("KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible")
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/svm/avic.c | 10 +---------
> > > 1 file changed, 1 insertion(+), 9 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > index 3c333cd2e752..14f567550a1e 100644
> > > --- a/arch/x86/kvm/svm/avic.c
> > > +++ b/arch/x86/kvm/svm/avic.c
> > > @@ -411,15 +411,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
> > > * 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;
> > > + int apic = ffs(bitmap) - 1;
> > >
> > > l1_physical_id = (cluster << 4) + apic;
> > > }
> >
> > Oh, I didn't notice this bug. However isn't removing the check is wrong as well?
> >
> > What if we do have multiple bits set in the bitmap? After you remove this code,
> > we will set IPI only to APIC which matches the 1st bit, no?
>
> The common code (out of sight here) already ensures the bitmap has exactly one bit set:
>
> if (unlikely(!bitmap))
> /* guest bug: nobody to send the logical interrupt to */
> return 0;
>
> if (!is_power_of_2(bitmap))
> /* multiple logical destinations, use slow path */
> return -EINVAL;
>
> logid_index = cluster + __ffs(bitmap);
>
Ah right, but again because the patch that added x2apic logic after I already added should not
be added. I vote again to just revert it.
Best regards,
Maxim Levitsky
