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

From: "Sjur Brændeland" <sjurbren@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
	Ohad Ben-Cohen <ohad@wizery.com>,
	Amit Shah <amit.shah@redhat.com>,
	linux-kernel@vger.kernel.org,
	Linus Walleij <linus.walleij@linaro.org>,
	virtualization@lists.linux-foundation.org
Subject: Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation
Date: Fri, 7 Sep 2012 11:24:24 +0200	[thread overview]
Message-ID: <CAJK669bqRF6WiQ8=21pEyUB565i80CeN0eniWcPBxXNLCOcxXA@mail.gmail.com> (raw)
In-Reply-To: <20120905191657.GA15868@redhat.com>

Hi Michael.

>> 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.
>
> how?
...
>> Even if we fix this particular problem, the general problem
>> still exists: bogus virtio declarations in remoteproc's firmware
>> may cause BUG_ON().
>
> which BUG_ON exactly?

I am afraid I have been barking up the wrong tree here.
Please ignore my previous rambling about panics related
to device features.

I hit the  BUG() in virtio_check_driver_offered_feature():
First I did not declare the feature because DMA was not set:
static unsigned int features[] = {
...
#if VIRTIO_CONSOLE_HAS_DMA
	VIRTIO_CONSOLE_F_DMA_MEM,
#endif
};

and then in probe I checked if the feature was supported:
	    virtio_has_feature(vdev, VIRTIO_CONSOLE_F_DMA_MEM)

This triggered the BUG() in virtio_check_driver_offered_feature(),
because the driver was asking for a unknown-feature.

I can get avoid this by simply checking the devices feature bits directly
instead of using virtio_has_feature().

Regards,
Sjur