On Wed, Sep 06, 2023, Xiaoyao Li wrote: > On 9/6/2023 2:24 PM, Hao Peng wrote: > > From: Peng Hao <flyingpeng@xxxxxxxxxxx> > > > > The call of vcpu_load/put takes about 1-2us. Each > > kvm_arch_vcpu_create will call vcpu_load/put > > to initialize some fields of vmcs, which can be > > delayed until the call of vcpu_ioctl to process > > this part of the vmcs field, which can reduce calls > > to vcpu_load. > > what if no vcpu ioctl is called after vcpu creation? > > And will the first (it was second before this patch) vcpu_load() becomes > longer? have you measured it? I don't think the first vcpu_load() becomes longer, this avoids an entire load()+put() pair by doing the initialization in the first ioctl(). That said, the patch is obviously buggy, it hooks kvm_arch_vcpu_ioctl() instead of kvm_vcpu_ioctl(), e.g. doing KVM_RUN, KVM_SET_SREGS, etc. will cause explosions. It will also break the TSC synchronization logic in kvm_arch_vcpu_postcreate(), which can "race" with ioctls() as the vCPU file descriptor is accessible by userspace the instant it's installed into the fd tables, i.e. userspace doesn't have to wait for KVM_CREATE_VCPU to complete. And I gotta imagine there are other interactions I haven't thought of off the top of my head, e.g. the vCPU is also reachable via kvm_for_each_vcpu(). All it takes is one path that touches a lazily initialized field for this to fall apart. > I don't think it worth the optimization unless a strong reason. Yeah, this is a lot of subtle complexity to shave 1-2us.
