• Paolo Bonzini's avatar
    KVM: race-free exit from KVM_RUN without POSIX signals · 460df4c1
    Paolo Bonzini authored
    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
    Reviewed-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
    Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    460df4c1
api.txt 134 KB