Re: [RFC] allow multi-core guests: introduce cores= option to -cpu

From: Andre Przywara <andre.przywara@amd.com>
To: Brian Jackson <iggy@theiggy.com>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org
Subject: Re: [RFC] allow multi-core guests: introduce cores= option to -cpu
Date: Sat, 4 Jul 2009 00:52:30 +0200	[thread overview]
Message-ID: <4A4E8BAE.7090602@amd.com> (raw)
In-Reply-To: <4A4E20BA.2040100@theiggy.com>

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.

Regards,
Andre.

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