2017-08-10 15:16-0300, Eduardo Habkost: > 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).
Ah, thanks. > 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. Good point, looks like it all comes to x86_cpu_apic_id_from_index(). Each level of the topology has at most one underutilized bit, so 2^(32 - 3) would be safe. It is still needlessly large for the foreseeable future, but 512 is going to be surpassed pretty soon, so I think that jumping at least to 8k would be better. (8k the current default maximum for Linux and the resulting overcommit of ~20 is bearable for smoke testing on current hardware.)
