On 15/02/2017 16:24, Christian Borntraeger wrote: > 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) Note that this series doesn't work (due to David's suggestion) with the patches I posted last week. Paolo
