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SubjectRe: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping
On Thu, Jun 30, 2016 at 03:10:11AM -0600, Jan Beulich wrote:
> >>> On 29.06.16 at 18:27, <andrew.cooper3@citrix.com> wrote:
> > On 29/06/16 17:19, Vitaly Kuznetsov wrote:
> >> To explain better what I'm trying to suggest here please take a look at
> >> the attached patch. If we can guarantee long term that ACPI id always
> >> equals to Xen's idea of vCPU id this is probably the easiest way.
> >>
> >> -- Vitaly
> >
> > The code in hvmloader which sets up the MADT does:
> >
> > for ( i = 0; i < hvm_info->nr_vcpus; i++ )
> > {
> > memset(lapic, 0, sizeof(*lapic));
> > lapic->type = ACPI_PROCESSOR_LOCAL_APIC;
> > lapic->length = sizeof(*lapic);
> > /* Processor ID must match processor-object IDs in the DSDT. */
> > lapic->acpi_processor_id = i;
> > lapic->apic_id = LAPIC_ID(i);
> > lapic->flags = (test_bit(i, hvm_info->vcpu_online)
> > ? ACPI_LOCAL_APIC_ENABLED : 0);
> > lapic++;
> > }
> >
> > So relying on the acpi_processor_id does look to be reliable. That code
> > hasn't changed since 2007, and that was only a bugfix. I would go so
> > far as to say it is reasonable for us to guarantee this in the guest ABI.
>
> In fact - is there any other way a guest could learn the vCPU IDs
> of its CPUs in a reliable way? I don't think so, and hence this de
> facto already is part of the ABI; we should of course spell it out
> somewhere.

CCing Joao.

Joao worked (and I think he posted an RFC patchset?) where this is changed so
that the true hardware topology (core, thread, etc) is exposed. This is obviously
for cases where you want pinning.

I would hesistate to spell this out as an ABI..

P.S.
Which reminds me, Joao, you OK posting the patchset?
>
> Jan
>
>
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> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

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