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

Lege Wang
2024-07-04T02:27:24Z

hi,

> -----Original Message-----
> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, July 3, 2024 8:34 PM
> To: Lege Wang <lege.wang@jaguarmicro.com>
> Cc: virtio-comment@lists.linux.dev; vattunuru@marvell.com;
> ndabilpuram@marvell.com; parav@nvidia.com; Leo Liu
> <leo.liu@jaguarmicro.com>; Angus Chen <angus.chen@jaguarmicro.com>
> Subject: Re: [PATCH] VIRTIO_F_USED_EVENT_AUTO_DISABLE: add new used
> buffer notification suppression mechanism
>
> External Mail: This email originated from OUTSIDE of the organization!
> Do not click links, open attachments or provide ANY information unless you
> recognize the sender and know the content is safe.
>
>
> 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?
Unless driver enables device's used buffer notification, the device will
not send an interrupt.

Regards,
Xiaoguang Wang
>
>
>
> > >
> > >
> > > --
> > > MST