Re: [Xen-devel] [PATCH] x86/hvm: Extend HVM cpuid leaf with vcpu id

[Matt Wilson <msw@xxxxxxxxx>]

On Wed, Dec 10, 2014 at 10:36:18AM +0000, Andrew Cooper wrote:
> On 10/12/14 06:00, Matt Wilson wrote:
> >
> > That's not strictly true. You can use the processor index in ACPI.
> 
> The LAPIC objects in the MADT/APIC table?  That is still constructed
> with the broken LAPIC_ID = 2 * processor_id logic.

Have you checked on an EC2 instance (for example: C3, I2, R3)?

> > And the Xen vCPU index can be determined by the processor object IDs
> > in DSDT.
> 
> The processor object IDs have no requirement in the spec to be in order,
> or contiguous.

True - this is a weak heuristic today, so having a CPUID based
approach would be more robust. (To be clear, I'm not arguing against
the proposed patch - I just want to make sure that the justification
doesn't give people the wrong impression about the functional way to
derive Xen vCPU ID without it.)

> A DSDT processor object refers back to the MADT, which in turn still
> has the broken logic.

See above. :-)

> Also, any solution using ACPI tables is going to be an issue for PVH guests.

PVH guests have the same facilities available to them for vCPU
enumeration as PV guests, no?

--msw

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