On Wed, Jan 31, 2024 at 9:27 AM Zhao Liu <zhao1....@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 <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?
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 <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 > > > > >