Re: [qemu-devel]

[Thomas Huth]

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