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[Qemu-devel] Re: [RFC] allow multi-core guests: introduce cores= option

From: Andre Przywara
Subject: [Qemu-devel] Re: [RFC] allow multi-core guests: introduce cores= option to -cpu
Date: Sat, 4 Jul 2009 00:52:30 +0200
User-agent: Thunderbird (X11/20081105)

Brian Jackson wrote:
Andre Przywara wrote:
currently SMP guests happen to see <n> vCPUs as <n> different sockets.
Some guests (Windows comes to mind) have license restrictions and refuse
to run on multi-socket machines.
So lets introduce a "cores=" parameter to the -cpu option to let the user
specify the number of _cores_ the guest should see.
>> ...

Personally, I'd like to see it as an extra arg to the -smp option. We've seen too many people use -cpu incorrectly in #kvm, so we've gotten into the habit of telling people not to touch that option unless they know exactly what they are doing. Plus it seems odd to have to use -cpu foo when you just want more cpus, not a specific cpu.

Ok, I see your point. I simply used -cpu because of technical reasons (the core topology is reflected in CPUID, which -cpu cares about). But you are right, it does not belong here, -smp looks like a good candidate (IMO better than -numa). Or we use an abstract "-topology" for this, but this seems like overkill. So what about: "-smp 4,cores=2,threads=2[,sockets=1]" to inject 4 vCPUs in one package (automatically determined if omitted) with two cores and two threads/core? All parameters except the number of vCPUs would be optional, which would make this new format backwards compatible. Only we have to agree on the default topology: multi-socket like the current implementation or multi-core, which would mimic the most common SMP architecture today. It seems that the latter one causes less problems for guests.


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
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