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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Feb 20, 2024 at 05:16:37PM +0100, Gerd Hoffmann wrote:
> 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).

Ok, lets just mention something hand-wavy like

 "pending various upstream EDK2 fixes which will probably be
  in the 2024-05 release"


With regards,
Daniel
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