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

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Date:	Tue, 11 Sep 2012 01:11:11 +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 08:27:35AM +0300, Michael S. Tsirkin wrote:
> 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>
> 
> --

Rusty?

> 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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