Re: [PATCH] virtio-blk: allow toggling host cache between writeback and writethrough

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 03, 2012 at 03:19:37PM +0200, Paolo Bonzini wrote:
> This patch adds support for the new VIRTIO_BLK_F_CONFIG_WCE feature,
> which exposes the cache mode in the configuration space and lets the
> driver modify it.  The cache mode is exposed via sysfs.
> 
> Even if the host does not support the new feature, the cache mode is
> visible (thanks to the existing VIRTIO_BLK_F_WCE), but not modifiable.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

I note this has been applied but I think the userspace
API is a bit painful to use. Let's fix it before
it gets set in stone?

Some more minor nits below.
Also Cc lists from MAINTAINERS.

> ---
>  drivers/block/virtio_blk.c |   90 ++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/virtio_blk.h |    5 ++-
>  2 files changed, 91 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 693187d..5602505 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -397,6 +397,83 @@ static int virtblk_name_format(char *prefix, int index, char *buf, int buflen)
>  	return 0;
>  }
>  
> +static int virtblk_get_cache_mode(struct virtio_device *vdev)

Why are you converting u8 to int here?
All users convert it back ...
Also, this is not really "get cache mode" it's more of a
"writeback_enabled".


> +{
> +	u8 writeback;
> +	int err;
> +
> +	err = virtio_config_val(vdev, VIRTIO_BLK_F_CONFIG_WCE,
> +				offsetof(struct virtio_blk_config, wce),
> +				&writeback);
> +	if (err)
> +		writeback = virtio_has_feature(vdev, VIRTIO_BLK_F_WCE);
> +
> +	return writeback;
> +}
> +
> +static void virtblk_update_cache_mode(struct virtio_device *vdev)
> +{
> +	u8 writeback = virtblk_get_cache_mode(vdev);
> +	struct virtio_blk *vblk = vdev->priv;
> +
> +	if (writeback)
> +		blk_queue_flush(vblk->disk->queue, REQ_FLUSH);
> +	else
> +		blk_queue_flush(vblk->disk->queue, 0);
> +
> +       revalidate_disk(vblk->disk);
> +}
> +
> +static const char *virtblk_cache_types[] = {
> +	"write through", "write back"
> +};
> +

I wonder whether something that lacks space would have been better,
especially for show: shells might get confused and split
a string at a space. How about we change it to writethrough,
writeback before it's too late? It's part of a userspace API
after all, and you see to prefer writeback in one word in your own
code so let's not inflict pain on others :)

Also, would be nice to make it discoverable what the legal
values are. Another attribute valid_cache_types with all values
space separated?


> +static ssize_t
> +virtblk_cache_type_store(struct device *dev, struct device_attribute *attr,
> +			 const char *buf, size_t count)
> +{
> +	struct gendisk *disk = dev_to_disk(dev);
> +	struct virtio_blk *vblk = disk->private_data;
> +	struct virtio_device *vdev = vblk->vdev;
> +	int i;
> +	u8 writeback;
> +
> +	BUG_ON(!virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_CONFIG_WCE));
> +	for (i = ARRAY_SIZE(virtblk_cache_types); --i >= 0; )
> +		if (sysfs_streq(buf, virtblk_cache_types[i]))
> +			break;
> +
> +	if (i < 0)
> +		return -EINVAL;
> +
> +	writeback = i;
> +	vdev->config->set(vdev,
> +			  offsetof(struct virtio_blk_config, wce),
> +			  &writeback, sizeof(writeback));
> +
> +	virtblk_update_cache_mode(vdev);
> +	return count;
> +}
> +
> +static ssize_t
> +virtblk_cache_type_show(struct device *dev, struct device_attribute *attr,
> +			 char *buf)
> +{
> +	struct gendisk *disk = dev_to_disk(dev);
> +	struct virtio_blk *vblk = disk->private_data;
> +	u8 writeback = virtblk_get_cache_mode(vblk->vdev);
> +
> +	BUG_ON(writeback >= ARRAY_SIZE(virtblk_cache_types));
> +	return snprintf(buf, 40, "%s\n", virtblk_cache_types[writeback]);

Why 40? Why snprintf? A plain sprintf won't do?

> +}
> +
> +static const struct device_attribute dev_attr_cache_type_ro =
> +	__ATTR(cache_type, S_IRUGO,
> +	       virtblk_cache_type_show, NULL);
> +static const struct device_attribute dev_attr_cache_type_rw =
> +	__ATTR(cache_type, S_IRUGO|S_IWUSR,
> +	       virtblk_cache_type_show, virtblk_cache_type_store);
> +
>  static int __devinit virtblk_probe(struct virtio_device *vdev)
>  {
>  	struct virtio_blk *vblk;
> @@ -474,8 +549,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	vblk->index = index;
>  
>  	/* configure queue flush support */
> -	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
> -		blk_queue_flush(q, REQ_FLUSH);
> +	virtblk_update_cache_mode(vdev);
>  
>  	/* If disk is read-only in the host, the guest should obey */
>  	if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
> @@ -553,6 +627,14 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	if (err)
>  		goto out_del_disk;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_BLK_F_CONFIG_WCE))
> +		err = device_create_file(disk_to_dev(vblk->disk),
> +					 &dev_attr_cache_type_rw);
> +	else
> +		err = device_create_file(disk_to_dev(vblk->disk),
> +					 &dev_attr_cache_type_ro);
> +	if (err)
> +		goto out_del_disk;
>  	return 0;
>  
>  out_del_disk:
> @@ -655,7 +737,7 @@ static const struct virtio_device_id id_table[] = {
>  static unsigned int features[] = {
>  	VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX, VIRTIO_BLK_F_GEOMETRY,
>  	VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE, VIRTIO_BLK_F_SCSI,
> -	VIRTIO_BLK_F_FLUSH, VIRTIO_BLK_F_TOPOLOGY
> +	VIRTIO_BLK_F_WCE, VIRTIO_BLK_F_TOPOLOGY, VIRTIO_BLK_F_CONFIG_WCE
>  };
>  
>  /*
> diff --git a/include/linux/virtio_blk.h b/include/linux/virtio_blk.h
> index e0edb40..18a1027 100644
> --- a/include/linux/virtio_blk.h
> +++ b/include/linux/virtio_blk.h
> @@ -37,8 +37,9 @@
>  #define VIRTIO_BLK_F_RO		5	/* Disk is read-only */
>  #define VIRTIO_BLK_F_BLK_SIZE	6	/* Block size of disk is available*/
>  #define VIRTIO_BLK_F_SCSI	7	/* Supports scsi command passthru */
> -#define VIRTIO_BLK_F_FLUSH	9	/* Cache flush command support */
> +#define VIRTIO_BLK_F_WCE	9	/* Writeback mode enabled after reset */
>  #define VIRTIO_BLK_F_TOPOLOGY	10	/* Topology information is available */
> +#define VIRTIO_BLK_F_CONFIG_WCE	11	/* Writeback mode available in config */
>  
>  #define VIRTIO_BLK_ID_BYTES	20	/* ID string length */
>  
> @@ -69,6 +70,8 @@ struct virtio_blk_config {
>  	/* optimal sustained I/O size in logical blocks. */
>  	__u32 opt_io_size;
>  
> +        /* writeback mode (if VIRTIO_BLK_F_CONFIG_WCE) */
> +        __u8 wce;
>  } __attribute__((packed));
>  
>  /*
> -- 
> 1.7.1
> 
> --
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