On Fri, 05 Nov 2021 20:21:36 +0000, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Fri, Nov 05, 2021, Marc Zyngier wrote: > > At least on arm64 and x86, the vcpus array is pretty huge (512 entries), > > and is mostly empty in most cases (running 512 vcpu VMs is not that > > common). This mean that we end-up with a 4kB block of unused memory > > in the middle of the kvm structure. > > Heh, x86 is now up to 1024 entries. Humph. I don't want to know whether people are actually using that in practice. The only time I create VMs with 512 vcpus is to check whether it still works... > > > Instead of wasting away this memory, let's use an xarray instead, > > which gives us almost the same flexibility as a normal array, but > > with a reduced memory usage with smaller VMs. > > > > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> > > --- > > @@ -693,7 +694,7 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i) > > > > /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu. */ > > smp_rmb(); > > - return kvm->vcpus[i]; > > + return xa_load(&kvm->vcpu_array, i); > > } > > It'd be nice for this series to convert kvm_for_each_vcpu() to use > xa_for_each() as well. Maybe as a patch on top so that potential > explosions from that are isolated from the initiali conversion? > > Or maybe even use xa_for_each_range() to cap at online_vcpus? > That's technically a functional change, but IMO it's easier to > reason about iterating over a snapshot of vCPUs as opposed to being > able to iterate over vCPUs as their being added. In practice I > doubt it matters. > > #define kvm_for_each_vcpu(idx, vcpup, kvm) \ > xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, atomic_read(&kvm->online_vcpus)) > I think that's already the behaviour of this iterator (we stop at the first empty slot capped to online_vcpus. The only change in behaviour is that vcpup currently holds a pointer to the last vcpu in no empty slot has been encountered. xa_for_each{,_range}() would set the pointer to NULL at all times. I doubt anyone relies on that, but it is probably worth eyeballing some of the use cases... Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm