Re: [PATCH v2] pc: q35: Bump max_cpus to 1024

[Daniel P . Berrangé]

On Thu, Jun 01, 2023 at 02:54:54PM -0700, Suthikulpanit, Suravee wrote:
> 
> 
> On 6/1/2023 6:09 PM, Michael S. Tsirkin wrote:
> > On Thu, Jun 01, 2023 at 11:17:30AM +0100, Daniel P. Berrangé wrote:
> > > On Thu, Jun 01, 2023 at 11:09:45AM +0100, Joao Martins wrote:
> > > > 
> > > > On 31/05/2023 23:51, Suravee Suthikulpanit wrote:
> > > > > Since KVM_MAX_VCPUS is currently defined to 1024 for x86 as shown in
> > > > > arch/x86/include/asm/kvm_host.h, update QEMU limits to the same 
> > > > > number.
> > > > > 
> > > > > In case KVM could not support the specified number of vcpus, QEMU 
> > > > > would
> > > > > return the following error message:
> > > > > 
> > > > >    qemu-system-x86_64: kvm_init_vcpu: kvm_get_vcpu failed (xxx): 
> > > > > Invalid argument
> > > > > 
> > > > > Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>
> > > > > ---
> > > > > 
> > > > > Changes from V1:
> > > > > (https://lore.kernel.org/all/YnkDGsIi1vFvXmiP@redhat.com/T/)
> > > > >   * Bump from 512 to KVM_MAX_VCPUS (per Igor's suggestion)
> > > > > 
> > > > > Note:
> > > > >   From the last discussion, Daniel mentioned that SMBIO 2.1 tables 
> > > > > might
> > > > >   cause overflow at approx 720 CPUs, and it might require using the
> > > > >   SMBIO 3.0 entry point. Also, we might need to change the default for
> > > > >   the x86 machine type to SMBIO 3.0. However, I do not know the status
> > > > >   of this.
> > > > > 
> > > > I suspect smbios 3.0 (64-bit entry point) is already supported.
> > > > 
> > > > With current qemu and all the smbios fixes in the last cycle, perhaps 
> > > > this is
> > > > mainly just setting smbios_entry_point_type to 
> > > > SMBIOS_ENTRY_POINT_TYPE_64 if
> > > > MachineState::smp::max_cpus is bigger than 720 (e.g. in pc_q35_init()?)
> > > >> The need for the 64-bit entry point depends on a combination of
> > > RAM
> config
> > > and CPU count. IMHO we need to unconditionally switch the latest machine
> > > types to use the 64-bit entry point by default, rather than trying to 
> > > infer
> > > some special condition to dynamically change on the fly.
> > Yes, makes sense.
> 
> Thanks all for the feedback. So, IIUC, here is how the SMBIOS entry point
> types would be affected by the QEMU options:
> 
> - pc-q35-8.1 and later, default to SMBIOS EP type 64.
> - pc-q35-8.0 and older, default to SMBIOS EP type 32.
> - User can override the type w/ QEMU option "-M ..,
> smbios-entry-point-type=[32|64]"
> 
> Please let me know if I am missing anything. If this is accurate, I'll send
> out v3 with this change.

Yes, and for pc-i440fx* too


With regards,
Daniel
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