The goal is to increase KVM_MAX_VCPUS without worrying about memory impact of many small guests. This is a second out of three major "dynamic" options: 1) size vcpu array at VM creation time 2) resize vcpu array when new VCPUs are created 3) use a lockless list/tree for VCPUs The disadvantage of (1) is its requirement on userspace changes and limited flexibility because userspace must provide the maximal count on start. The main advantage is that kvm->vcpus will work like it does now. It has been posted as "[PATCH 0/4] KVM: add KVM_CREATE_VM2 to allow dynamic kvm->vcpus array", http://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1377285.html The main problem of (2), this series, is that we cannot extend the array in place and therefore require some kind of protection when moving it. RCU seems best, but it makes the code slower and harder to deal with. The main advantage is that we do not need userspace changes. The third option wasn't explored yet. It would solve the ugly kvm_for_each_vcpu() of (2), but kvm_get_vcpu() would become linear. (We could mitigate it by having list of vcpu arrays and A lockless sequentially growing "tree" would be logarithmic and not that much more complicated to implement.) Which option do you think is the best? Thanks. Radim Krčmář (2): KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC KVM: RCU protected dynamic vcpus array arch/mips/kvm/mips.c | 8 +++-- arch/powerpc/kvm/powerpc.c | 6 ++-- arch/s390/kvm/kvm-s390.c | 27 ++++++++++++----- arch/x86/kvm/hyperv.c | 3 +- arch/x86/kvm/vmx.c | 3 +- arch/x86/kvm/x86.c | 5 ++-- include/linux/kvm_host.h | 73 +++++++++++++++++++++++++++++----------------- virt/kvm/arm/arm.c | 10 +++---- virt/kvm/kvm_main.c | 72 +++++++++++++++++++++++++++++++++++++-------- 9 files changed, 144 insertions(+), 63 deletions(-) -- 2.13.3
