Re: [PATCH v5 01/45] virtio: use u32, not bitmap for struct virtio_device's features

David Hildenbrand
2014-11-27T16:15:42Z

> From: Rusty Russell <rusty@rustcorp.com.au>
>
> It seemed like a good idea, but it's actually a pain when we get more
> than 32 feature bits. Just change it to a u32 for now.
>
> Cc: Brian Swetland <swetland@google.com>
> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> Acked-by: Pawel Moll <pawel.moll@arm.com>
> Acked-by: Ohad Ben-Cohen <ohad@wizery.com>
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---

Wouldn't it be easier to turn
test_bit(i, dev->features)

into a simple makro like
#define test_bit(i, features) (features & (1 << i))

Similar one for clear_bit and set_bit.

if names collide with existing functions, we could simply rename them to:

set_feature_bit() ...
clear_feature_bit() ...

> diff --git a/drivers/misc/mic/card/mic_virtio.c b/drivers/misc/mic/card/mic_virtio.c
> index e647947..0acd564 100644
> --- a/drivers/misc/mic/card/mic_virtio.c
> +++ b/drivers/misc/mic/card/mic_virtio.c
> @@ -101,7 +101,7 @@ static void mic_finalize_features(struct virtio_device *vdev)
> bits = min_t(unsigned, feature_len,
> sizeof(vdev->features)) * 8;
> for (i = 0; i < bits; i++) {
> - if (test_bit(i, vdev->features))
> + if (vdev->features & BIT(bits))

Hm, shouldn't that also be "if (vdev->features & (1 << i))"?

> iowrite8(ioread8(&out_features[i / 8]) | (1 << (i % 8)),
> &out_features[i / 8]);
> }

> out_free:
> kfree(features);
> kfree(ccw);
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index df598dd..7814b1f 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -49,9 +49,9 @@ static ssize_t features_show(struct device *_d,
>
> /* We actually represent this as a bitstring, as it could be
> * arbitrary length in future. */
> - for (i = 0; i < ARRAY_SIZE(dev->features)*BITS_PER_LONG; i++)
> + for (i = 0; i < sizeof(dev->features)*8; i++)

... sizeof(dev->features) * 8; ...

Do we have a define for 8 ?

> len += sprintf(buf+len, "%c",
> - test_bit(i, dev->features) ? '1' : '0');
> + dev->features & (1ULL << i) ? '1' : '0');
> len += sprintf(buf+len, "\n");
> return len;
> }
> @@ -168,18 +168,18 @@ static int virtio_dev_probe(struct device *_d)
> device_features = dev->config->get_features(dev);
>

[...]

> @@ -483,7 +483,7 @@ int main(int argc, char *argv[])
>
> /* Set up host side. */
> vring_init(&vrh.vring, RINGSIZE, __user_addr_min, ALIGN);
> - vringh_init_user(&vrh, vdev.features[0], RINGSIZE, true,
> + vringh_init_user(&vrh, vdev.features, RINGSIZE, true,
> vrh.vring.desc, vrh.vring.avail, vrh.vring.used);
>
> /* No descriptor to get yet... */
> @@ -652,13 +652,13 @@ int main(int argc, char *argv[])
> }
>
> /* Test weird (but legal!) indirect. */
> - if (vdev.features[0] & (1 << VIRTIO_RING_F_INDIRECT_DESC)) {
> + if (vdev.features & (1 << VIRTIO_RING_F_INDIRECT_DESC)) {
> char *data = __user_addr_max - USER_MEM/4;
> struct vring_desc *d = __user_addr_max - USER_MEM/2;
> struct vring vring;
>
> /* Force creation of direct, which we modify. */
> - vdev.features[0] &= ~(1 << VIRTIO_RING_F_INDIRECT_DESC);
> + vdev.features &= ~(1 << VIRTIO_RING_F_INDIRECT_DESC);

That could then be something like

clear_feature_bit(VIRTIO_RING_F_INDIRECT_DESC, vdev.features);

(I would prefer such makros over repeating bit actions)

> vq = vring_new_virtqueue(0, RINGSIZE, ALIGN, &vdev, true,
> __user_addr_min,
> never_notify_host,

But on the whole this looks good to me.