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Re: [Qemu-devel] [PATCH 2/3] push CPUID level to 4 to allow Intel multic
Re: [Qemu-devel] [PATCH 2/3] push CPUID level to 4 to allow Intel multicore decoding
Thu, 20 Aug 2009 12:36:18 +0200
Thunderbird 188.8.131.52 (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
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.
AMD-Operating System Research Center (OSRC), Dresden, Germany
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[Qemu-devel] [PATCH 1/3] extend -smp parsing to include cores= and threads= options, Andre Przywara, 2009/08/19
[Qemu-devel] [PATCH 3/3] set CPUID bits to present cores and threads topology, Andre Przywara, 2009/08/19