linux-kernel - Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

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Message-ID: <20120906052735.GB17656@redhat.com>
Date:	Thu, 6 Sep 2012 08:27:35 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Sjur Brændeland <sjurbren@...il.com>,
	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

On Thu, Sep 06, 2012 at 11:34:25AM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> > On Tue, Sep 04, 2012 at 06:58:47PM +0200, Sjur Brændeland wrote:
> >> Hi Michael,
> >> 
> >> > Exactly. Though if we just fail load it will be much less code.
> >> >
> >> > Generally, using a feature bit for this is a bit of a problem though:
> >> > normally driver is expected to be able to simply ignore
> >> > a feature bit. In this case driver is required to
> >> > do something so a feature bit is not a good fit.
> >> > I am not sure what the right thing to do is.
> >> 
> >> I see - so in order to avoid the binding between driver and device
> >> there are two options I guess. Either make virtio_dev_match() or
> >> virtcons_probe() fail. Neither of them seems like the obvious choice.
> >> 
> >> Maybe adding a check for VIRTIO_CONSOLE_F_DMA_MEM match
> >> between device and driver in virtcons_probe() is the lesser evil?
> >> 
> >> Regards,
> >> Sjur
> >
> > A simplest thing to do is change dev id. rusty?
> 
> For generic usage, this is correct.  But my opinion is that fallback on
> feature non-ack is quality-of-implementation issue: great to have, but
> there are cases where you just want to fail with "you're too old".
> 
> And in this case, an old system simply will never work.  So it's a
> question of how graceful the failure is.
> 
> Can your userspace loader can refuse to proceed if the driver doesn't
> ack the bits?  If so, it's simpler than a whole new ID.
> 
> Cheers,
> Rusty.

Yes but how can it signal guest that it will never proceed?

Also grep for BUG_ON in core found this:

       drv->remove(dev);

       /* Driver should have reset device. */
       BUG_ON(dev->config->get_status(dev));

I think below is what Sjur refers to.
I think below is a good idea for 3.6. Thoughts?

--->

virtio: don't crash when device is buggy

Because of a sanity check in virtio_dev_remove, a buggy device can crash
kernel.  And in case of rproc it's userspace so it's not a good idea.
We are unloading a driver so how bad can it be?
Be less aggressive in handling this error: if it's a driver bug,
warning once should be enough.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

--

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index c3b3f7f..1e8659c 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -159,7 +159,7 @@ static int virtio_dev_remove(struct device *_d)
 	drv->remove(dev);
 
 	/* Driver should have reset device. */
-	BUG_ON(dev->config->get_status(dev));
+	WARN_ON_ONCE(dev->config->get_status(dev));
 
 	/* Acknowledge the device's existence again. */
 	add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);

-- 
MST
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