Re: [PATCH v3 2/6] KVM: X86: Implement PV IPIs in linux guest

[Wanpeng Li <kernellwp@gmail.com>]

On Fri, 20 Jul 2018 at 00:47, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 19/07/2018 18:28, Radim Krčmář wrote:
> >> +
> >> +    kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap_low, ipi_bitmap_high, vector);
> > and
> >
> >       kvm_hypercall3(KVM_HC_SEND_IPI, ipi_bitmap[0], ipi_bitmap[1], vector);
> >
> > Still, the main problem is that we can only address 128 APICs.
> >
> > A simple improvement would reuse the vector field (as we need only 8
> > bits) and put a 'offset' in the rest.  The offset would say which
> > cluster of 128 are we addressing.  24 bits of offset results in 2^31
> > total addressable CPUs (we probably should even use that many bits).
> > The downside of this is that we can only address 128 at a time.
> >
> > It's basically the same as x2apic cluster mode, only with 128 cluster
> > size instead of 16, so the code should be a straightforward port.
> > And because x2apic code doesn't seem to use any division by the cluster
> > size, we could even try to use kvm_hypercall4, add ipi_bitmap[2], and
> > make the cluster size 192. :)
>
> I did suggest an offset earlier in the discussion.
>
> The main problem is that consecutive CPU ids do not map to consecutive
> APIC ids.  But still, we could do an hypercall whenever the total range
> exceeds 64.  Something like
>
> u64 ipi_bitmap = 0;
> for_each_cpu(cpu, mask)
>         if (!ipi_bitmap) {
>                 min = max = cpu;
>         } else if (cpu < min && max - cpu < 64) {
>                 ipi_bitmap <<= min - cpu;
>                 min = cpu;
>         } else if (id < min + 64) {
>                 max = cpu < max ? max : cpu;
>         } else {
>                 /* ... send hypercall... */
>                 min = max = cpu;
>                 ipi_bitmap = 0;
>         }
>         __set_bit(ipi_bitmap, cpu - min);
> }
> if (ipi_bitmap) {
>         /* ... send hypercall... */
> }
>
> We could keep the cluster size of 128, but it would be more complicated
> to do the left shift in the first "else if".  If the limit is 64, you
> can keep the two arguments in the hypercall, and just pass 0 as the
> "high" bitmap on 64-bit kernels.

As David pointed out, we need to scale to higher APIC IDs. I will add
the cpu id to apic id transfer in the for loop. How about:
kvm_hypercall2(KVM_HC_SEND_IPI, ipi_bitmap, vector); directly. In
addition, why need to pass the 0 as the "high" bitmap even if for 128
vCPUs case?

Regards,
Wanpeng Li