Re: [Xen-devel] [PATCH v3 11/13] xen/arm: send IPIs to inject irqs into guest vcpus running on different pcpus

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-04-24 at 20:07 +0100, Stefano Stabellini wrote:
> If we need to inject an irq into a guest that is running on a different

                                     ^VCPU

> processor, we shouldn't just enqueue the irq into the lr_pending and
> inflight lists and wait for something to interrupt the guest execution.
> Send an IPI to the target pcpu so that Xen can inject the new interrupt
> returning to guest.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> ---
>  xen/arch/arm/vgic.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
> index 8912aab..547a2ab 100644
> --- a/xen/arch/arm/vgic.c
> +++ b/xen/arch/arm/vgic.c
> @@ -664,6 +664,8 @@ out:
>      spin_unlock_irqrestore(&v->arch.vgic.lock, flags);
>      /* we have a new higher priority irq, inject it into the guest */
>      vcpu_unblock(v);
> +    if ( v != current && v->is_running )
> +        smp_send_event_check_mask(cpumask_of(v->processor));

I'm a bit concerned about races here, e.g. where v is busy going to
sleep. I don't know enough about the behaviour of the scheduler and
vcpu_unblock etc to say, but it worries me...

x86 has vcpu_kick which saves v->is_running before the unblock and then
sends a softirq with much the same checks as you have -- that seems like
a plausible race avoidance strategy, although whether IPI vs softirq is
an important distinction I've no idea...

BTW smp_send_event_check_mask(cpumask_of(v->processor)) is the same as
smp_send_event_check_cpu(v->processor).

Ian.


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