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

[Daniel P . Berrangé]

On Tue, Jan 30, 2024 at 10:39:51PM +0530, 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.

Yes, could you resend with 1856, and include a description of
the blocking problem in the commit message for the historical
record, as this is the kind of thing that people will have
forgotten when re-visiting the patch later and wondering why
this limit was chosen.


With regards,
Daniel
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