On Sat, Sep 14, 2013 at 01:13:59PM +0100, Marc Zyngier wrote: > On 2013-09-14 12:58, Andrew Jones wrote: > >On Fri, Sep 13, 2013 at 01:46:59PM +0100, Marc Zyngier wrote: > >>On 11/09/13 14:02, Anup Patel wrote: > >>> Current max VCPUs per-Guest is set to 4 which is preventing > >>> us from creating a Guest (or VM) with 8 VCPUs on Host (e.g. > >>> X-Gene Storm SOC) with 8 Host CPUs. > >>> > >>> The correct value of max VCPUs per-Guest should be same as > >>> the max CPUs supported by GICv2 which is 8 hence this patch > >>> increases KVM_MAX_VCPUS to 8. > >> > >>If anything, please make it configurable just like we have on > >>32bit. No > >>reason to impose the extra overhead on everyone. > > > >What type of overhead are we talking about? Memory, right? as > > Memory indeed. > > >kvm_for_each_vcpu is almost always used when iterating. But Anup > >says in > >his v2 of this patch "can make things slower". If it's memory, > >then is so > >much consumed by each vcpu that we shouldn't always set KVM_MAX_VCPUS > > Not only. See how the GIC emulation also has per-VCPU data, and this > results in potentially huge data structures. I have plans to address > this in the future though. > > >to at least the highest number that current hardware supports? > >Particularly > >for aarch64 I think we should always be considering multi-platform > >with the > >kernel configs. > > What do you mean by multiplatform? This constant has nothing to do > with being multiplatform, and the initial commit message was quite > misleading in this respect. You can perfectly have 8 VCPUs on a > single CPU host, and nothing so far impacts multiplatform. I meant compile the kernel once, but still have it useful on multiple hosts (some with 4 cpus, some with 8...). Of course with config option that's still possible, just compile with CONFIG_KVM_MAX_VCPUS=8. So I guess this is really more of a debate about what the default should be. Considering the GIC code may be a bit of an over-consumer at the moment and 8-cpu hosts may not be that widely deployed, then maybe 4 is the better default right now? drew _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/cucslists/listinfo/kvmarm
