Re: [qemu-devel]

[Pawel Polawski]

Hi Thomas,

Thank you for the suggestion about the next step.

Do you know if there is an option to change this parameter in the runtime?

While looking for answer I found this presentation from 2008 which makes me think that this may not

be the only limit: https://www.linux-kvm.org/images/b/be/KvmForum2008%24kdf2008_6.pdf

I will try to change this value in the kernel code, rebuild it and see if it works.

PS: Just realized that my original message has split for some reason and there are two copies on the mailing list,

each with different subjects.

Best,

Pawel

On Tue, Nov 15, 2022 at 8:44 AM Thomas Huth <thuth@redhat.com> wrote:

On 14/11/2022 23.58, Pawel Polawski wrote:
> Hi Everyone,
>
> I am trying to check qemu virtual cpu boundaries when running a custom
> edk2 based firmware build. For that purpose I want to run qemu with more
> than 1024 vCPU:
> $QEMU
> -accel kvm
> -m 4G
> -M q35,kernel-irqchip=on,smm=on
> -smp cpus=1025,maxcpus=1025 -global mch.extended-tseg-mbytes=128
> -drive if=pflash,format=raw,file=${CODE},readonly=on
> -drive if=pflash,format=raw,file=${VARS}
> -chardev stdio,id=fwlog
> -device isa-debugcon,iobase=0x402,chardev=fwlog "$@"
>
> The result is as follows:
> QEMU emulator version 7.0.50 (v7.0.0-1651-g9cc1bf1ebc-dirty)
> Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers
> qemu-system-x86_64: -accel kvm: warning: Number of SMP cpus requested (1025)
> exceeds the recommended cpus supported by KVM (8)
> Number of SMP cpus requested (1025) exceeds the maximum cpus supported by
> KVM (1024)
>
> It is not clear to me if I am hitting qemu limitation or KVM limitation here.
> I have changed hardcoded 1024 limits in hw/i386/* files but the limitation
> is still presented.
>
> Can someone advise what I should debug next looking for those vCPU limits?

Well, the error message says it: There is a limitation in KVM, i.e. in the
kernel code, too. I think it is KVM_MAX_VCPUS in the file
arch/x86/include/asm/kvm_host.h of the Linux kernel sources... so if you're
brave, you might want to increase that value there and rebuild your own
kernel. Not sure whether that works, though.

  Thomas

--

Paweł Poławski

Red Hat Virtualization

ppolawsk@redhat.com   

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