Re: [Xen-devel] Re: [PATCH] libxc: remove CPUID core information mangling

[Andre Przywara <andre.przywara@xxxxxxx>]

Keir Fraser wrote:
> Ah yes, I agree.
>
>
> On 25/08/2010 16:53, "Huang2, Wei" <Wei.Huang2@xxxxxxx> wrote:
>
> > OK. BTW, the old way seems wrong. The correct implementation should be
> > (((regs[2] & 0xf000u) >> 12) + 1) << 12.
> >
> > -----Original Message-----
> > From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> > Sent: Wednesday, August 25, 2010 10:39 AM
> > To: Huang2, Wei; Przywara, Andre; Nitin Kamble
> > Cc: xen-devel
> > Subject: Re: [Xen-devel] Re: [PATCH] libxc: remove CPUID core information
> > mangling
> >
> > I meant it should remain the old way, since HVM virtual APIC IDs are
> > vcpu_id*2.
I agree, that seems to be best way for the time being. Although this 
value is actually meant to tell different processors apart, so I guess 
it needs a revisit later.
FYI:
Real machines use different ways to assign APIC-IDs, for example my 
4-way Magny-Cours (4*12 cores) has this:
0x00-0x02: used for I/O-APICs, (could be 4-bit constrained)
    -0x0f: reserved for IOAPICs
0x10-0x1b: LAPIC-IDs for cores from the 1st processor
0x20-0x2b: LAPIC-IDs for cores from the 2nd processor
0x30-0x3b: LAPIC-IDs for cores from the 3rd processor
0x40-0x4b: LAPIC-IDs for cores from the 4th processor
The 80000008:ECX[15:12] value is 4, which means the lower 4 bits of the 
LAPIC ID indicate the core number within each package.
Obviously this scheme does not fit the Xen one's.

Thanks Wei for spotting the calculation error!

Regards,
Andre.

> >  -- Keir
> >
> > On 25/08/2010 16:28, "Huang2, Wei" <Wei.Huang2@xxxxxxx> wrote:
> >
> > > Hi Keir,
> > >
> > > Do you mean that we should leave 80000008:ECX[15:12] as zero or in old way
> > > (i.e. (regs[2] & 0xf000u) + 1))? These bits can't be zero, unless we want to
> > > use legacy method in multi-core calculation.
> > >
> > > -Wei
> > >
> > > ========
> > > I think you shouldn't change handling of 80000008:ECX[15:12] since that does
> > > explicitly refer to APIC ID arrangement. The rest of your changes could be
> > > correct as far as I can tell from the reference manuals.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12


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