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

From: Christian Borntraeger
Date: Wed Feb 15 2017 - 10:24:29 EST


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)