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Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals

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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@redhat.com>


Generic parts, the concept and the s390 parts looks good. (not tested yet, though)

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[PATCH] KVM: race-free exit from KVM_RUN without POSIX signals Paolo Bonzini <pbonzini@redhat.com> - 2017-02-15 15:50 +0100
  Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals Christian Borntraeger <borntraeger@de.ibm.com> - 2017-02-15 16:30 +0100
    Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals Paolo Bonzini <pbonzini@redhat.com> - 2017-02-15 17:00 +0100
  Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals Radim Krčmář <rkrcmar@redhat.com> - 2017-02-16 17:30 +0100
  Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals David Hildenbrand <david@redhat.com> - 2017-02-16 20:30 +0100
    Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals Paolo Bonzini <pbonzini@redhat.com> - 2017-02-17 10:50 +0100

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