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

[Zhao Liu <zhao1.liu@intel.com>]

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?

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
> 
>