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