Re: [PATCH] pc: q35: Bump max_cpus to 512

[Daniel P . Berrangé]

On Tue, May 10, 2022 at 09:03:25AM +0200, Igor Mammedov wrote:
> On Mon, 9 May 2022 13:03:38 +0100
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> > On Mon, May 09, 2022 at 09:12:49AM +0200, Igor Mammedov wrote:
> > > On Wed, 4 May 2022 08:16:39 -0500
> > > Suravee Suthikulpanit <suravee.suthikulpanit@amd.com> wrote:
> > >   
> > > > This is the maximum number of vCPU supported by
> > > > the AMD x2APIC virtualization.
> > > > 
> > > > Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> > > > ---
> > > >  hw/i386/pc_q35.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > > > index 302288342a..e82b1c690d 100644
> > > > --- a/hw/i386/pc_q35.c
> > > > +++ b/hw/i386/pc_q35.c
> > > > @@ -357,7 +357,7 @@ static void pc_q35_machine_options(MachineClass *m)
> > > >      machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
> > > >      machine_class_allow_dynamic_sysbus_dev(m, TYPE_RAMFB_DEVICE);
> > > >      machine_class_allow_dynamic_sysbus_dev(m, TYPE_VMBUS_BRIDGE);
> > > > -    m->max_cpus = 288;
> > > > +    m->max_cpus = 512;  
> > > 
> > > Maybe we should bump it to KVM VCPU maximum,
> > > and make sure we error out if asked for combination of
> > > hardware/irqchip is not usable.  
> > 
> > In RHEL downstream we currently bump this to 710 CPUs, because you
> > overflow the SMBIOS 2.1 tables at approx 720 CPUs (give/take a little
> > depending on other config options).
> 
> Also there were some testing done with 1024,
> but my main reason for matching KVM's limit is unblock upstream
> testing so it would be easier to push limits for others.
> Downstream can clamp that value down to whatever it deems as supported.
> 
> > Going beyond 710 CPUs value requires using the SMBIOS 3 entry point.
> > 
> > AFAIK, the x86 machine types still default to SMBIOS 2.1, so that
> > would need changing too.
> 
> Yep, we can change default to SMBIOS 3 starting with new machine type (7.1?)
> or conditionally depending on requested number of CPUs,
> though I'd prefer machine type approach.

Agree, machine type is better IMHO, a it gives us a consistent guest
ABI regardless of CPU count.

> As for SMBIOS 3, we still have to update CPU structures to support more than
> 255 vcpus (Julia was working on it). It's long standing bug, but that doesn't
> seem to be critical, as guest boots fine with old structures.

What's the impact of SMBIOS 3 being limited to 255 ?  That's lower than
the current max CPUs of 288 in upstream / 710 in downstream.


With regards,
Daniel
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