Re: Qemu virtual CPU limitation to 1024

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
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From:	Pawel Polawski
Subject:	Re: Qemu virtual CPU limitation to 1024
Date:	Tue, 15 Nov 2022 09:49:34 +0100