On 07/21/2011 02:20 AM, John Paul Walters wrote:
Hi,
We have a 256 core SGI Ultraviolet machine running RHEL 6.1 with qemu-kvm 0.13, and we'd like to be able to start large guest VMs of up to 256 cores. I see that x86 guests are currently limited to 64 VCPUs. Is there any reason for this hard limitation? It appears that we can't get around this limitation by simply redefining the kernel's KVM_MAX_VCPUS to 256. Qemu-kvm and possibly SeaBIOS seem to require changes as well. Can anyone offer any suggestions as to how straightforward it would be to increase the number of CPUs that we can allocate to KVM guests?
And here I am on record saying no one wants this...
kvm.git has patches increasing the limit to 254 (256 is not possible due
to the APIC ID being 8 bits and two IDs being reserved).
Latest seabios appears to have no cpu limits; qemu is limited to 255.
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I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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