linux-kernel - Re: [PATCH] KVM: race-free exit from KVM

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Date:   Wed, 15 Feb 2017 16:24:12 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, christoffer.dall@...aro.org,
        marc.zyngier@....com, james.hogan@...tec.com, paulus@...ba.org,
        cornelia.huck@...ibm.com, kvmarm@...ts.cs.columbia.edu,
        kvm-ppc@...r.kernel.org
Subject: Re: [PATCH] KVM: race-free exit from KVM_RUN without POSIX signals

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@...hat.com>


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