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

Michael S. Tsirkin
2012-07-04T15:42:13Z

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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