Re: [Qemu-devel] [PATCH 2/3] push CPUID level to 4 to allow Intel multicore decoding

[Andre Przywara]

Jamie Lokier wrote:
> Andre Przywara wrote:
> > The other option would be to push the level only to four if we use more 
> > than one thread or core.
> >
> > In my research it turned out that Intel pushed the level beyond 4
> > with Pentium4 Prescott (probably with the introduction of real dual
> > core chips to differentiate threads and cores), so this is quite
> > some time ago.
>
> Pentium 4 wasn't that long ago, from a point of view of running legacy
> guests.
Well, but long enough to get those software issues ruled out. If any 
serious OS would have had problems, somebody would have noticed and 
fixed it.

> I see quite a lot of code in Linux (to pick an example - presumably
> also other OSes) which checks for things by looking at model and
> family number, so I wonder if it's wise to depart a long way from
> CPUIDs which resemble real hardware which has existed.
Yeah, but most of the stuff Linux checks are bugs and workarounds, so I 
am not 100% sure whether it's an advantage. We should be careful with 
adjusting the working default QEMU/KVM has right now.

> We've already seen a problem along these lines, which was the x86
> CPUID_SEP feature not getting used properly, but that was quite well
> hidden, only affecting one application (Skype) on Windows.
>
> Could it make more sense to, in effect, upgrade the default CPUID
> to Pentium 4 (instead of Pentium Pro) when cores > 1?
OK, OK, I will finally send this patch out (following in another mail).
This is a kvm64 CPU (which should become the new KVM's default, if it 
proves to work well). It looks like a P4 Prescott, that seems to be a 
good base, since it has a decent base line of features and is among the 
first CPUs which are supported by KVM.
If someone has such a real beast (best would be a Pentium 4 662 or other 
Prescotts with x86_64, HT and VT-x), I would love to see a complete 
CPUID dump (including the 0000_0004 subleafs, which x86info omits).

Regards,
Andre.

--
Andre Przywara
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