linux-kernel - Re: [PATCH v5 01/45] virtio: use u32, not bitmap for struct virtio

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Date:	Thu, 27 Nov 2014 17:15:42 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	cornelia.huck@...ibm.com, rusty@....ibm.com, nab@...ux-iscsi.org,
	pbonzini@...hat.com, thuth@...ux.vnet.ibm.com,
	Rusty Russell <rusty@...tcorp.com.au>,
	Brian Swetland <swetland@...gle.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Pawel Moll <pawel.moll@....com>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Amit Shah <amit.shah@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ashutosh Dixit <ashutosh.dixit@...el.com>,
	Sudeep Dutt <sudeep.dutt@...el.com>,
	Siva Yerramreddy <yshivakrishna@...il.com>,
	Joel Stanley <joel@....id.au>,
	virtualization@...ts.linux-foundation.org, lguest@...ts.ozlabs.org,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH v5 01/45] virtio: use u32, not bitmap for struct
 virtio_device's features

> From: Rusty Russell <rusty@...tcorp.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@...gle.com>
> Cc: Christian Borntraeger <borntraeger@...ibm.com>
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
> Signed-off-by: Cornelia Huck <cornelia.huck@...ibm.com>
> Acked-by: Pawel Moll <pawel.moll@....com>
> Acked-by: Ohad Ben-Cohen <ohad@...ery.com>
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.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.

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