Subject: Re: [PATCH 1/2] virito: introduce methods of fixing device features

From: Jason Wang <[email-masked]>
Date: Thu, 13 Nov 2014 17:11:30 +0800

On 11/13/2014 04:46 PM, Cornelia Huck wrote:
> On Thu, 13 Nov 2014 13:52:53 +0800
> Jason Wang <[email-masked]> wrote:
>
> typo in subject-prefix: s/virito/virtio/
>

Will correct this.
>> Buggy host may advertised buggy host features (a usual case is that host
>> advertise a feature whose dependencies were missed). In this case, driver
>> should detect and disable the buggy features by itself.
>>
>> This patch introduces driver specific fix_features() method which is called
>> just before features finalizing to detect and disable buggy features
>> advertised by host.
> So the basic problem this patch fixes is that an individual driver may
> only specify a static set of features but cannot specify any
> dependencies, right?

Right, and what even worse is qemu could not handle dependencies as
well. So we need fix both sides.
>  Adding a sanitizer step makes sense, I guess.
>
>> Virtio-net will be the first user.
>>
>> Cc: Rusty Russell <[email-masked]>
>> Cc: Michael S. Tsirkin <[email-masked]>
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>  drivers/virtio/virtio.c       |  4 ++++
>>  include/linux/virtio.h        |  1 +
>>  include/linux/virtio_config.h | 12 ++++++++++++
>>  3 files changed, 17 insertions(+)
>>
>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
>> index df598dd..7001d6e 100644
>> --- a/drivers/virtio/virtio.c
>> +++ b/drivers/virtio/virtio.c
>> @@ -181,6 +181,10 @@ static int virtio_dev_probe(struct device *_d)
>>  		if (device_features & (1 << i))
>>  			set_bit(i, dev->features);
>>
>> +	/* Fix buggy features advertised by host */
>> +	if (drv->fix_features)
>> +		drv->fix_features(dev);
> I'd probably call this "sanitize_features" instead.

Ok.
>> +
>>  	dev->config->finalize_features(dev);
>>
>>  	err = drv->probe(dev);
>> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
>> index 7f4ef66..7bd89ea 100644
>> --- a/include/linux/virtio_config.h
>> +++ b/include/linux/virtio_config.h
>> @@ -96,6 +96,18 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
>>  	return test_bit(fbit, vdev->features);
>>  }
>>
>> +static inline void virtio_disable_feature(struct virtio_device *vdev,
>> +                                          unsigned int fbit)
>> +{
>> +	BUG_ON(fbit >= VIRTIO_TRANSPORT_F_START);
>> +	BUG_ON(vdev->config->get_status(vdev) &
>> +	       ~(VIRTIO_CONFIG_S_ACKNOWLEDGE | VIRTIO_CONFIG_S_DRIVER));
> When we add virtio-1 support, we can add a check for FEATURES_OK here,
> so we're really on the safe side.
>

If I read the spec correctly, FEATURES_OK was set only after writing the
features bits to device. But we want to sanitize the them before.
>> +
>> +	virtio_check_driver_offered_feature(vdev, fbit);
>> +
>> +	clear_bit(fbit, vdev->features);
>> +}
>> +
>>  static inline
>>  struct virtqueue *virtio_find_single_vq(struct virtio_device *vdev,
>>  					vq_callback_t *c, const char *n)
> The approach looks good to me.
>

Thanks for the review.