Re: [PATCH] VIRTIO_F_USED_EVENT_AUTO_DISABLE: add new used buffer notification suppression mechanism

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 03, 2024 at 12:14:01PM +0000, Lege Wang wrote:
> hi,
> 
> > > > After reading this and the text, I can't figure out how is this feature
> > > > supposed to work without races.  What if the device uses a buffer while
> > > > the driver notification is in flight to the device?
> > > Hmm, I think it doesn't matter, Only the used buffer enable notification is
> > > transmitted to device successfully, device starts to judge whether it's time
> > > to send used buffer notification to driver according to its internal interrupt
> > policy.
> > 
> > I don't understand what you are saying. If device has some magical
> > interrupt policy why do we need a notification from the driver
> > at all.
> > 
> > Conversely, if device relies on the notification and thinks that an
> > interrupt is not needed (because it did not get the notification yet),
> > while at the same time the driver is blocked waiting for an interrupt,
> > we get a deadlock.
> No, I'm not saying that our interrupt policy is magical?? Let me explain a bit
> more about its simple mechanism here, there're two basic parameters to control
> whether device should send used buffer notification to driver:
>   1. number of used ring entries written since last used buffer notification to driver, 
>     say threshold value is N.
>   2. Max wait time to send a used buffer notification to driver.
> 
> The work flow would look like below:
> 1. driver enables used buffer notification initially when enabling virtio device
> 2. driver is blocked waiting interrupt.
> 3. if the number of used ring entries written by device is equal to or greater than above threshold N,
>   a used buffered notification is sent to driver immediately, or if this condition is not met, but the period
>   specified by max wait time has reached, device will still send one used buffer notification to driver.
>   And device will recount number of used ring entries written by device from zero and start another
>   Max wait time timer.
> 4. driver is waken up by interrupt, after it has processed used ring entries, it'll start to wait for interrupt
>   again and enable used buffer notification.
> 5. go to step 2.
> 
> >From above work flow, I don't think the deadlock you describe would happen.
> 
> Regards,
> Xiaoguang Wang

1. device sends an interrupt
2. device writes a used entry

Will another interrupt be sent, with driver doing nothing at all?



> > 
> > 
> > --
> > MST