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
