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

Cool, i will try the offset method in next version. Thanks for your
review, Paolo! :)

Regards,
Wanpeng Li

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