[PATCH/RFC] virtio_blk: check for hardsector size from host

Christian Borntraeger
2008-05-27T09:05:13Z

Rusty, Jens,

I need your opinion on the following patch. It seems to work, but I would like
to get some feedback if this patch is the right approach:

---

Currently virtio_blk assumes a 512 byte hard sector size. This can cause
trouble / performance issues if the backing has a different block size
(like a file on an ext3 file system formatted with 4k block size or a dasd
device).

Lets add a feature flag that tells the guest to use a different hard sector
size than 512 byte. The host can use stat->st_blksize to determine the right
hard sector size for a specific backing.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
drivers/block/virtio_blk.c | 10 +++++++++-
include/linux/virtio_blk.h | 3 +++
2 files changed, 12 insertions(+), 1 deletion(-)

Index: kvm/drivers/block/virtio_blk.c
===================================================================
--- kvm.orig/drivers/block/virtio_blk.c
+++ kvm/drivers/block/virtio_blk.c
@@ -196,6 +196,7 @@ static int virtblk_probe(struct virtio_d
int err;
u64 cap;
u32 v;
+ u64 blk_size;

if (index_to_minor(index) >= 1 << MINORBITS)
return -ENOSPC;
@@ -290,6 +291,13 @@ static int virtblk_probe(struct virtio_d
if (!err)
blk_queue_max_hw_segments(vblk->disk->queue, v);

+ /* Host can optionally specify the block size of the device */
+ err = virtio_config_val(vdev, VIRTIO_BLK_F_BLK_SIZE,
+ offsetof(struct virtio_blk_config, blk_size),
+ &blk_size);
+ if (!err)
+ blk_queue_hardsect_size(vblk->disk->queue, blk_size);
+
add_disk(vblk->disk);
return 0;

@@ -329,7 +337,7 @@ static struct virtio_device_id id_table[

static unsigned int features[] = {
VIRTIO_BLK_F_BARRIER, VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX,
- VIRTIO_BLK_F_GEOMETRY, VIRTIO_BLK_F_RO,
+ VIRTIO_BLK_F_GEOMETRY, VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE,
};

static struct virtio_driver virtio_blk = {
Index: kvm/include/linux/virtio_blk.h
===================================================================
--- kvm.orig/include/linux/virtio_blk.h
+++ kvm/include/linux/virtio_blk.h
@@ -11,6 +11,7 @@
#define VIRTIO_BLK_F_SEG_MAX 2 /* Indicates maximum # of segments */
#define VIRTIO_BLK_F_GEOMETRY 4 /* Legacy geometry available */
#define VIRTIO_BLK_F_RO 5 /* Disk is read-only */
+#define VIRTIO_BLK_F_BLK_SIZE 6 /* Block size of disk is available*/

struct virtio_blk_config
{
@@ -26,6 +27,8 @@ struct virtio_blk_config
__u8 heads;
__u8 sectors;
} geometry;
+ /* block size of device (if VIRTIO_BLK_F_BLK_SIZE) */
+ __u64 blk_size;
} __attribute__((packed));

/* These two define direction. */