Re: [PATCH 07/19] KVM: SVM: Drop buggy and redundant AVIC "single logical dest" check

[Sean Christopherson <seanjc@google.com>]

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@google.com>
> > ---
> >  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);