Re: [patch] qemu-kvm introduce -maxcpus argument

[Jes Sorensen <jes.sorensen@gmail.com>]

On 06/19/2009 04:15 PM, Daniel P. Berrange wrote:
> On Fri, Jun 19, 2009 at 03:59:57PM +0200, Jes Sorensen wrote:
>> Hi,
>>
>> This one introduces a -maxcpus setting, allowing the user to specify
>> the maximum number of vCPUs the system can have, as discussed with Gleb
>> earlier in the week.
>
> What is the maximum value for the 'maxcpus' setting for KVM ?

Right now it is still 16, however I plan to change this.

> libvirt currently does
>
>      fd = open("/dev/kvm")
>      r = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
>
> to figure out what the maximum allowed vCPUs will be for KVM,
> and currently it is returning 16 IIRC.

Interesting, this will need to be addressed as well. I have plans to
introduce a mechanism telling the kernel where the limit will be, in
order to allow it to allocate data structures in a reasonable manner.

>> @@ -5666,6 +5667,13 @@ int main(int argc, char **argv, char **e
>>                       exit(1);
>>                   }
>>                   break;
>> +            case QEMU_OPTION_maxcpus:
>> +                max_cpus = atoi(optarg);
>> +                if ((max_cpus<  1) || (max_cpus>  machine->max_cpus)) {
>> +                    fprintf(stderr, "Invalid number of CPUs\n");
>> +                    exit(1);
>> +                }

> This implies the limit (for x86 pc machine at least) is now 255. Is that
> the correct interpretation on my part ?

Actually the 255 limit is tied to ACPI. Once we support ACPI 3.0 and
x2apic, it will get much worse. Be afraid, be very afraid :-) To be
honest, that is going to be a while before we get to that, but I hope
to get to it eventually.

I strongly recommend you try not to impose static limits within libvirt
for the number of vCPUs.

I guess it will become a tricky issue who is telling who what the limit
is. Ideally I would like to see the kernel limit becoming unlimited and
the restrictions being set by userland.

Cheers,
Jes