Re: [PATCH v3] pc: q35: Bump max_cpus to 4096 vcpus

[Gerd Hoffmann]

On Tue, Feb 20, 2024 at 03:50:57PM +0000, Daniel P. Berrangé wrote:
> On Tue, Feb 20, 2024 at 09:12:04PM +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. At present, QEMU has been tested to correctly boot a
> > linux guest with 4096 vcpus both with edk2 and seabios firmwares.
> > So bump up the value max_cpus to 4096 for q35 machines versions 9 and newer.
> > Q35 machines versions 8.2 and older continue to support 1024 maximum vcpus
> > as before for compatibility reasons.
> > 
> > If KVM is not able to support the specified number of vcpus, QEMU would
> > return the following error messages:
> > 
> > $ ./qemu-system-x86_64 -cpu host -accel kvm -machine q35 -smp 1728
> > qemu-system-x86_64: -accel kvm: warning: Number of SMP cpus requested 
> > (1728) exceeds the recommended cpus supported by KVM (12)
> > qemu-system-x86_64: -accel kvm: warning: Number of hotpluggable cpus 
> > requested (1728) exceeds the recommended cpus supported by KVM (12)
> > Number of SMP cpus requested (1728) exceeds the maximum cpus supported by 
> > KVM (1024)
> > 
> > Cc: Daniel P. Berrangé <berrange@redhat.com>
> > Cc: Igor Mammedov <imammedo@redhat.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Julia Suvorova <jusual@redhat.com>
> > Cc: kraxel@redhat.com
> > Signed-off-by: Ani Sinha <anisinha@redhat.com>
> > ---
> >  hw/i386/pc_q35.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > Changelog:
> > v3: bump up to 4096 vcpus. It has now been tested to work with edk2.
> > See RH Jira: https://issues.redhat.com/browse/RHEL-22202
> 
> That bug indicates a dependancy on a EDK2 patch
> 
>   
> https://github.com/kraxel/edk2/commit/7a03c17f0f4f4a9003d77db2660c8e087604b2f0
> 
> we'll need to rebase the EDK2 ROMs in QEMU to get that included.

Which will btw take a while.  edk2 is in freeze for the 2024-02 release
right now, I expect the changes land upstream shortly thereafter and
will be part of the 2024-05 release.  So end may / early june would be
the time when rebasing to release, or somewhen in march or april when we
rebase to a git snapshot ...

> Meanwhile, plesae at least call out this EDK2 commit as a
> pre-requisite in the commit message, so people know the
> EDK2 ROMs in QEMU won't work (yet).

That surely makes sense.

Oh, and it's more than just that one commit.  I don't think it makes
sense to compile a list of commits given this is a moving target
(upstream review is in progress right now).

take care,
  Gerd