Re: [PATCH v5] pc: q35: Bump max_cpus to 4096 vcpus

[Michael S. Tsirkin]

On Tue, Mar 12, 2024 at 12:04:37PM +0530, Ani Sinha wrote:
> 
> 
> > On 28-Feb-2024, at 20:03, Ani Sinha <anisinha@redhat.com> 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 using the current edk2 upstream master branch 
> > that
> > has the fixes corresponding to the following two PRs:
> > 
> > https://github.com/tianocore/edk2/pull/5410
> > https://github.com/tianocore/edk2/pull/5418
> > 
> > The changes merged into edk2 with the above PRs will be in the upcoming 
> > 2024-05
> > release. With current seabios firmware, it boots fine with 4096 vcpus 
> > already.
> > 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
> > Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> > Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> > Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> > Signed-off-by: Ani Sinha <anisinha@redhat.com>
> 
> Ping .. who is picking this up? The soft code freeze starts today?


I did.

> > ---
> > hw/i386/pc_q35.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > Changelog:
> > v5: mention the PRs in the commit message. Add tags.
> > v4: tweaked commit message as per suggestion from danpb explicitly
> > stating that 4096 vcpus work with edk2 fixes that are going to be
> > available in the coming edk2 release.
> > 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
> > v2: bump up the vcpu number to 1856. Add failure messages from ekd2 in
> > the commit description.
> > 
> > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > index 45a4102e75..df63a92b78 100644
> > --- a/hw/i386/pc_q35.c
> > +++ b/hw/i386/pc_q35.c
> > @@ -350,7 +350,7 @@ static void pc_q35_machine_options(MachineClass *m)
> >     m->default_nic = "e1000e";
> >     m->default_kernel_irqchip_split = false;
> >     m->no_floppy = 1;
> > -    m->max_cpus = 1024;
> > +    m->max_cpus = 4096;
> >     m->no_parallel = !module_object_class_by_name(TYPE_ISA_PARALLEL);
> >     machine_class_allow_dynamic_sysbus_dev(m, TYPE_AMD_IOMMU_DEVICE);
> >     machine_class_allow_dynamic_sysbus_dev(m, TYPE_INTEL_IOMMU_DEVICE);
> > @@ -371,6 +371,7 @@ static void pc_q35_8_2_machine_options(MachineClass *m)
> > {
> >     pc_q35_9_0_machine_options(m);
> >     m->alias = NULL;
> > +    m->max_cpus = 1024;
> >     compat_props_add(m->compat_props, hw_compat_8_2, hw_compat_8_2_len);
> >     compat_props_add(m->compat_props, pc_compat_8_2, pc_compat_8_2_len);
> > }
> > -- 
> > 2.42.0
> >