Re: [PATCH] pc: q35: Bump max_cpus to 1728 vcpus

[Philippe Mathieu-Daudé]

On 30/1/24 18:09, Ani Sinha wrote:
> > On 30-Jan-2024, at 22:17, Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > On Tue, Jan 30, 2024 at 10:14:28PM +0530, Ani Sinha wrote:
> > > Since commit f10a570b093e6 ("KVM: x86: Add CONFIG_KVM_MAX_NR_VCPUS to allow up to 
> > > 4096 vCPUs")
> > > Linux kernel can support upto a maximum number of 4096 vCPUS when MAXSMP is
> > > enabled in the kernel. QEMU has been tested to correctly boot a linux guest
> > > with 1728 vcpus both with edk2 and seabios firmwares. So bump up the max_cpus
> > > value for q35 machines versions 9 and newer to 1728. Q35 machines versions
> > > 8.2 and older continue to support 1024 maximum vcpus as before for
> > > compatibility.
> >
> > Where does the 1728 number come from ?
> >
> > Did something break at 1729, or did the test machine simply not
> > have sufficient resources to do practical larger tests ?
>
> Actual limit currently is 1856 for EDK2. The HPE folks tested QEMU with edk2 
> and QEMU fails to boot beyond that limit.
> There are RH internal bugs tracking this and Gerd is working on it from RH side 
> [1].
>
> We would ultimately like to go to 8192 vcpus for SAP HANA but 1728 vcpus is our 
> immediate target for now. If you want, I can resend the patch with 1856 since 
> that is currently the tested limit.
>
> 1. https://issues.redhat.com/browse/RHEL-22202
Out of curiosity has the limit to be multiple of 64?