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 <anisi...@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 <berra...@redhat.com>
> Cc: Igor Mammedov <imamm...@redhat.com>
> Cc: Michael S. Tsirkin <m...@redhat.com>
> Cc: Julia Suvorova <jus...@redhat.com>
> Cc: kra...@redhat.com
> Signed-off-by: Ani Sinha <anisi...@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
>
>
