Re: [PATCH] pc: q35: Bump max_cpus to 1728 vcpus

[Michael S. Tsirkin]

On Tue, Jan 30, 2024 at 10:39:51PM +0530, Ani Sinha wrote:
> 
> 
> > On 30-Jan-2024, at 22:17, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > 
> > On Tue, Jan 30, 2024 at 10:14:28PM +0530, Ani Sinha 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. QEMU has been tested to correctly boot a linux guest
> >> with 1728 vcpus both with edk2 and seabios firmwares. So bump up the 
> >> max_cpus
> >> value for q35 machines versions 9 and newer to 1728. Q35 machines versions
> >> 8.2 and older continue to support 1024 maximum vcpus as before for
> >> compatibility.
> > 
> > Where does the 1728 number come from ?
> > 
> > Did something break at 1729, or did the test machine simply not
> > have sufficient resources to do practical larger tests ?
> 
> Actual limit currently is 1856 for EDK2. The HPE folks tested QEMU with edk2 
> and QEMU fails to boot beyond that limit.
> There are RH internal bugs tracking this and Gerd is working on it from RH 
> side [1].
> 
> We would ultimately like to go to 8192 vcpus for SAP HANA but 1728 vcpus is 
> our immediate target for now. If you want, I can resend the patch with 1856 
> since that is currently the tested limit.
> 
> 1. https://issues.redhat.com/browse/RHEL-22202

What is requested here is that you document the source of the number whatever
it is in a code comment and commit log.


> 
> > 
> >> 
> >> 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>
> >> Signed-off-by: Ani Sinha <anisinha@redhat.com>
> >> ---
> >> hw/i386/pc_q35.c | 3 ++-
> >> 1 file changed, 2 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> >> index f43d5142b8..bfa627a70b 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 = 1728;
> >>     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
> >> 
> > 
> > With regards,
> > Daniel
> > -- 
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> > :|
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>