Re: [Qemu-devel] [PATCH] x86: Increase max vcpu number to 352

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Aug 10, 2017 at 02:41:03PM +0200, Radim Krčmář wrote:
> 2017-08-10 19:02+0800, Lan Tianyu:
> > On 2017年08月10日 18:26, Daniel P. Berrange wrote:
> >> On Thu, Aug 10, 2017 at 06:08:07PM +0800, Lan Tianyu wrote:
> >>> Intel Xeon phi chip will support 352 logical threads. For HPC
> >>> usage case, it will create a huge VM with vcpus number as same as host
> >>> cpus. This patch is to increase max vcpu number to 352.
> >> 
> >> If we pick arbitray limits based on size of physical CPUs that happen
> >> to be shipping today, we'll continue the cat+mouse game forever trailing
> >> latest CPUs that vendors ship.
> >> 
> >> IMHO we should pick a higher number influenced by technical constraints
> >> of the q35 impl instead. eg can we go straight to something like 512 or
> >> 1024  ?
> > 
> > Sure. 512 should be enough and some arrays is defined according to max
> > vcpu number.
> 
> Hm, which arrays are that?  I was thinking it is safe to bump it to
> INT_MAX as the number is only used when setting global max_cpus.

We had a MAX_CPUMASK_BITS macro, and bitmaps whose sizes were
defined at compile time based on it.  But commit
cdda2018e3b9ce0c18938767dfdb1e05a05b67ca removed it.  Probably
those arrays all use max_cpus, by now (and the default for
max_cpus is smp_cpus, not MachineClass::max_cpus).

Anyway, if we set it to INT_MAX, there are some cases where more
appropriate error checking/reporting could be required because
they won't handle overflow very well:
* pcms->apic_id_limit initialization at pc_cpus_init()
* ACPI code that assumes possible_cpus->cpus[i].arch_id fits
  in a 32-bit integer
* Other x86_cpu_apic_id_from_index() calls in PC code
  (especially the initialization of possible_cpus->cpus[i].arch_id).
  Note that x86_cpu_apic_id_from_index(cpu_index) might not fit
  in 32 bits even if cpu_index <= UINT32_MAX.

-- 
Eduardo