On 02/15/2017 03:43 PM, Paolo Bonzini wrote: > The purpose of the KVM_SET_SIGNAL_MASK API is to let userspace "kick" > a VCPU out of KVM_RUN through a POSIX signal. A signal is attached > to a dummy signal handler; by blocking the signal outside KVM_RUN and > unblocking it inside, this possible race is closed: > > VCPU thread service thread > -------------------------------------------------------------- > check flag > set flag > raise signal > (signal handler does nothing) > KVM_RUN > > However, one issue with KVM_SET_SIGNAL_MASK is that it has to take > tsk->sighand->siglock on every KVM_RUN. This lock is often on a > remote NUMA node, because it is on the node of a thread's creator. > Taking this lock can be very expensive if there are many userspace > exits (as is the case for SMP Windows VMs without Hyper-V reference > time counter). > > As an alternative, we can put the flag directly in kvm_run so that > KVM can see it: > > VCPU thread service thread > -------------------------------------------------------------- > raise signal > signal handler > set run->immediate_exit > KVM_RUN > check run->immediate_exit > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Generic parts, the concept and the s390 parts looks good. (not tested yet, though)
