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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 12:36:18 +0200
User-agent: Thunderbird (X11/20090329)

Avi Kivity wrote:
On 08/19/2009 04:42 PM, Andre Przywara wrote:
Intel CPUs store the number of cores in CPUID leaf 4. So push
the maxleaf value to 4 to allow the guests access to this leaf.

There's a slight compatibility risk here. If a guest has broken handling for cpuid level 4, then upgrading qemu would cause it to behave differently.

I don't think that's an issue for this patch, just highlighting the need for a systematic treatment of backward compatibility.

If you have real headaches about it, I have two options:

What about allowing level to be specified at -cpu command line? This would allow users to say -cpu qemu64,level=2 if they experience problems. The default would stay at level 4.

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. So I doubt that there are serious issues out there. The only problem I can think of is that the advertised cache topology is somehow bogus and could confuse OSes.


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