Re: [PATCH 5/5] KVM: Convert the kvm->vcpus array to a xarray
From: Sean Christopherson <seanjc@google.com>
Date: 2021-11-05 20:22:28
Also in:
kvm, kvmarm, linuxppc-dev
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.
quoted hunk ↗ jump to hunk
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@kernel.org> ---@@ -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))