Qemu virtual CPU limitation to 1024

Qemu virtual CPU limitation to 1024

[Pawel Polawski]

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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?

Best regards,
Pawel


-- 

Paweł Poławski

Red Hat <https://www.redhat.com/> Virtualization

ppolawsk@redhat.com
@RedHat <https://twitter.com/redhat>   Red Hat
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Re: Qemu virtual CPU limitation to 1024

[Daniel P. Berrangé]

On Tue, Nov 15, 2022 at 12:19:35AM +0100, 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.

As per the error message, this is a KVM limit. QEMU queries the KVM
kernel module for this limit at runtime.


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
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Re: Qemu virtual CPU limitation to 1024

[Pawel Polawski]

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Hi Daniel,

Thank you for the answer.
Also I realized that somehow my email has split into two different threads,
sorry for the confusion.

Best,
Pawel

On Tue, Nov 15, 2022 at 9:48 AM Daniel P. Berrangé <berrange@redhat.com>
wrote:

> On Tue, Nov 15, 2022 at 12:19:35AM +0100, 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.
>
> As per the error message, this is a KVM limit. QEMU queries the KVM
> kernel module for this limit at runtime.
>
>
> With regards,
> Daniel
> --
> |: https://berrange.com      -o-
> https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-
> https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-
> https://www.instagram.com/dberrange :|
>
>

-- 

Paweł Poławski

Red Hat <https://www.redhat.com/> Virtualization

ppolawsk@redhat.com
@RedHat <https://twitter.com/redhat>   Red Hat
<https://www.linkedin.com/company/red-hat>  Red Hat
<https://www.facebook.com/RedHatInc>
<https://red.ht/sig>

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