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

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Date:	Wed, 5 Sep 2012 20:15:36 +0200
From:	Sjur Brændeland <sjurbren@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ohad Ben-Cohen <ohad@...ery.com>
Cc:	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	virtualization@...ts.linux-foundation.org,
	Sjur Brændeland <sjur.brandeland@...ricsson.com>
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

Hi Michael.

>> If the device then asks for VIRTIO_CONSOLE_F_DMA_MEM
>> when DMA is not supported, virtio will do BUG_ON() from
>> virtio_check_driver_offered_feature().
>>
>> Is this acceptable or should we add a check in virtcons_probe()
>> and let the probing fail instead?
>>
>> E.g:
>>       /* Refuse to bind if F_DMA_MEM request cannot be met */
>>       if (!VIRTIO_CONSOLE_HAS_DMA &&
>>           (vdev->config->get_features(vdev) & (1 << VIRTIO_CONSOLE_F_DMA_MEM))){
>>               dev_err(&vdev->dev,
>>                       "DMA_MEM requested, but arch does not support DMA\n");
>>               err = -EINVAL;
>>               goto fail;
>>       }
>>
>> Regards,
>> Sjur
>
> Failing probe would be cleaner. But there is still a problem:
> old driver will happily bind to that device and then
> fail to work, right?

Not just fail to work, the kernel will panic on the BUG_ON().
Remoteproc gets the virtio configuration from firmware loaded
from user space. So this type of problem might be triggered
for other virtio drivers as well.

> virtio pci has revision id for this, but remoteproc doesn't
> seem to have anything similar. Or did I miss it?

No there are currently no sanity check of
virtio type and feature bits in remoteproc.
One option may be to add this...

> If not -
> we probably need to use a different
> device id, and not a feature bit.

But if I create a new virtio console type, remoteproc
could still call the existing virtio_console with random
bad feature bits, causing kernel panic.

Even if we fix this particular problem, the general problem
still exists: bogus virtio declarations in remoteproc's firmware
may cause BUG_ON(). (Note the fundamental difference
between visualizations and remoteproc. For remoteproc
the virtio configuration comes from binaries loaded from
user space).

So maybe we should look for a more generic solution, e.g.
changing virtio probe functionality so that devices with
bad feature bits will not trigger BUG_ON(), but rather refuse
to bind the driver.

Regards,
Sjur
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