Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

[Joao Martins]

On 07/05/2016 04:44 PM, Jan Beulich wrote:
>>>> On 05.07.16 at 17:34, <konrad.wilk@xxxxxxxxxx> wrote:
>> On Thu, Jun 30, 2016 at 03:10:11AM -0600, Jan Beulich wrote:
>>>>>> On 29.06.16 at 18:27, <andrew.cooper3@xxxxxxxxxx> 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..
> 
> Are you perhaps mixing up ACPI and APIC IDs? Here talk is of the
> former, while Joao's patch set was about the latter iirc.
> 
Correct, my patch series indeed changed APIC IDs. I guess making acpi_processor_id to
get vcpu_id reliably stated as guest ABI, is safe wrt to future arrangements to apic_ids.

Joao