Re: [PATCH v2] pc: q35: Bump max_cpus to 1856 vcpus

[Ani Sinha]

On Wed, Jan 31, 2024 at 9:27 AM Zhao Liu <zhao1.liu@intel.com> wrote:
>
> Hi Ani,
>
> On Wed, Jan 31, 2024 at 08:19:06AM +0530, Ani Sinha wrote:
> > Date: Wed, 31 Jan 2024 08:19:06 +0530
> > From: Ani Sinha <anisinha@redhat.com>
> > Subject: [PATCH v2] pc: q35: Bump max_cpus to 1856 vcpus
> > X-Mailer: git-send-email 2.42.0
> >
> > 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 1856 vcpus and no more both with edk2 and seabios 
> > firmwares.
>
> About background, could I ask if there will be Host machines with so
> much CPUs? What are the benefits of vCPUs that far exceed the number
> of Host CPUs?

Yes HPE has SAP HANA host machines with large numbers of physical
cores and memory. For example QEMU was tested on a system with 3840
cores.

>
> Thanks,
> Zhao
>
> > If an additional vcpu is added, that is with 1857 vcpus, edk2 currently 
> > fails
> > with the following error messages:
> >
> > AllocatePages failed: No 0x400 Pages is available.
> > There is only left 0x2BF pages memory resource to be allocated.
> > ERROR: Out of aligned pages
> > ASSERT 
> > /builddir/build/BUILD/edk2-ba91d0292e/MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.c(814):
> >  BigPageAddress != 0
> >
> > This error exists only with edk2. Seabios currently can boot a linux guest
> > fine with 4096 vcpus. Since the lowest common denominator for a working VM 
> > for
> > both edk2 and seabios is 1856 vcpus, bump up the value max_cpus to 1856 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:
> > v2: bump up the vcpu number to 1856. Add failure messages from ekd2 in
> > the commit description.
> > See also RH Jira https://issues.redhat.com/browse/RHEL-22202
> >
> > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > index f43d5142b8..f9c4b6594d 100644
> > --- a/hw/i386/pc_q35.c
> > +++ b/hw/i386/pc_q35.c
> > @@ -375,7 +375,7 @@ 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 = 1856;
> >      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);
> > @@ -396,6 +396,7 @@ 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
> >
> >
>