Re: [Xen-devel] Why xen-pirq chip use startup_irq() for .irq_enable?

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

(Adjusting addressees: David is no longer maintaining Xen code, Juergen is)

On 07/27/2017 09:04 AM, shuo.a.liu@xxxxxxxxx wrote:
> Hi,
> Here is a device has xen-pirq-MSI interrupt. I found dom0 might lost
> interrupt during driver irq_disable/irq_enable.
> There is a pair of irq_disable/enable in driver. Here is the scenario,
>  1. irq_disable(dev_irq) -> disable_dynirq -> mask_evtchn(dev_irq
> channel)
>  2. dev interrupt raised by HW and Xen mark its evtchn as *pending*
> status.
>  3. irq_enable(dev_irq) -> startup_pirq -> eoi_pirq ->
>     clear_evtchn(channel of dev_irq) -> clear *pending* status
>  4. consume_one_event process the dev irq event without pending bit
> assert
>     which result in interrupt lost once.
>  5. No HW interrupt raising anymore.
>
> The first question here is why using startup_irq for .irq_enable
> rather than
> enable_dynirq ? startup_irq will do eoi_pirq who clear the mask bit
> and pending
> bit of the channel while enable_dynirq just only unmask the channel.

Seems like enable_dynirq() would indeed be the right choice. What is a
bit strange is that scenario that you are describing looks pretty common
so we should have hit this problem before.

>
> Second question is that what's the purpose of eoi_pirq in startup_irq?

When we are actually creating new pirq we want to make sure there are no
pending interrupts left over from previous use of the pirq.

-boris

>
> BTW, i can resolve my problem by below patch. Does it make sence?
>
> ---
> drivers/xen/events/events_base.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/xen/events/events_base.c
> b/drivers/xen/events/events_base.c
> index 4bf7a34..341c456 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -582,7 +582,7 @@ static void shutdown_pirq(struct irq_data *data)
>
> static void enable_pirq(struct irq_data *data)
> {
> -    startup_pirq(data);
> +    enable_dynirq(data);
> }
>
> static void disable_pirq(struct irq_data *data)


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