On 08/08/2011 10:18 PM, John Paul Walters wrote:
On Jul 21, 2011, at 2:10 AM, Avi Kivity wrote: > 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. > Hi again, I've applied the 254 core patches (below) from kvm.git on a RHEL 6.1 kernel. The new modules build and insert fine. https://git.kernel.org/?p=virt/kvm/kvm.git;a=commit;h=052fa7f4c5e79262cffcdc90bdd94172e00d45e3 https://git.kernel.org/?p=virt/kvm/kvm.git;a=commit;h=29a07f8e31980599c586ea7d1f84957bc7fe98ed However, whenever I try to boot a system with more than 83 CPUs, the system fails to boot with: Booting from Hard Disk... Boot failed: could not read the boot disk I'm using qemu-kvm.git with the following command line: /opt/qemu.git/bin/qemu-system-x86_64 -smp 84 -hda big_image_2.qcow2 -m 8388 -redir tcp:52109::22 Does anyone have any suggestions?
Most likely a seabios failure. Suggest you enable debugging in seabios and see what's going on; also copy the seabios mailing list.
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