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[Xen-devel] [PATCH v3] xen-blk(front|back): Handle large physical sector disks

Changed the xenbus_gather but kept the xenbus_dev_error (without being fatal) as
that seems closer to the style of the other calls.


From cddea87d842cc5feb427f3eedefc0566d69fb9b6 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Date: Mon, 13 May 2013 16:28:15 +0200
Subject: [PATCH] xen/blk: Use physical sector size for setup

Currently xen-blkback passes the logical sector size over xenbus and
xen-blkfront sets up the paravirt disk with that logical block size.
But newer drives usually have the logical sector size set to 512 for
compatibility reasons and would show the actual sector size only in
physical sector size.
This results in the device being partitioned and accessed in dom0 with
the correct sector size, but the guest thinks 512 bytes is the correct
block size. And that results in poor performance.

To fix this, blkback gets modified to pass also physical-sector-size
over xenbus and blkfront to use both values to set up the paravirt
disk. I did not just change the passed in sector-size because I am
not sure having a bigger logical sector size than the physical one
is valid (and that would happen if a newer dom0 kernel hits an older
domU kernel). Also this way a domU set up before should still be
accessible (just some tools might detect the unaligned setup).

[v2: Make xenbus write failure non-fatal]
[v3: Use xenbus_scanf instead of xenbus_gather]

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
 drivers/block/xen-blkback/xenbus.c |    5 +++++
 drivers/block/xen-blkfront.c       |   23 +++++++++++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
index 8bfd1bc..25463e3 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -704,6 +704,11 @@ again:
                goto abort;
+       err = xenbus_printf(xbt, dev->nodename, "physical-sector-size", "%u",
+                           bdev_physical_block_size(be->blkif->vbd.bdev));
+       if (err)
+               xenbus_dev_error(dev, err, "writing %s/physical-sector-size",
+                                dev->nodename);

        err = xenbus_transaction_end(xbt, 0);
        if (err == -EAGAIN)
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index d89ef86..33b5618 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -542,7 +542,8 @@ wait:

-static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
+static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size,
+                               unsigned int physical_sector_size)
        struct request_queue *rq;
        struct blkfront_info *info = gd->private_data;
@@ -564,6 +565,7 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16

        /* Hard sector size and max sectors impersonate the equiv. hardware. */
        blk_queue_logical_block_size(rq, sector_size);
+       blk_queue_physical_block_size(rq, physical_sector_size);
        blk_queue_max_hw_sectors(rq, 512);

        /* Each segment in a request is up to an aligned page in size. */
@@ -667,7 +669,8 @@ static char *encode_disk_name(char *ptr, unsigned int n)

 static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
                               struct blkfront_info *info,
-                              u16 vdisk_info, u16 sector_size)
+                              u16 vdisk_info, u16 sector_size,
+                              unsigned int physical_sector_size)
        struct gendisk *gd;
        int nr_minors = 1;
@@ -734,7 +737,7 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
        gd->driverfs_dev = &(info->xbdev->dev);
        set_capacity(gd, capacity);

-       if (xlvbd_init_blk_queue(gd, sector_size)) {
+       if (xlvbd_init_blk_queue(gd, sector_size, physical_sector_size)) {
                goto release;
@@ -1395,6 +1398,7 @@ static void blkfront_connect(struct blkfront_info *info)
        unsigned long long sectors;
        unsigned long sector_size;
+       unsigned int physical_sector_size;
        unsigned int binfo;
        int err;
        int barrier, flush, discard, persistent;
@@ -1437,6 +1441,16 @@ static void blkfront_connect(struct blkfront_info *info)

+       /*
+        * physcial-sector-size is a newer field, so old backends may not
+        * provide this. Assume physical sector size to be the same as
+        * sector_size in that case.
+        */
+       err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
+                          "physical-sector-size", "%u", &physical_sector_size);
+       if (err != 1)
+               physical_sector_size = sector_size;
        info->feature_flush = 0;
        info->flush_op = 0;

@@ -1483,7 +1497,8 @@ static void blkfront_connect(struct blkfront_info *info)
                info->feature_persistent = persistent;

-       err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
+       err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size,
+                                 physical_sector_size);
        if (err) {
                xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",

Attachment: 0001-xen-blk-Use-physical-sector-size-for-setup.patch
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