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

From: Andre Przywara
Subject: Re: [Qemu-devel] [PATCH 2/3] push CPUID level to 4 to allow Intel multicore decoding
Date: Thu, 20 Aug 2009 23:35:07 +0200
User-agent: Thunderbird (X11/20081105)

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
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).


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 488-3567-12
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