From 1832a90928232cb91a8542613b754079fd1f0f0e Mon Sep 17 00:00:00 2001

From: Max Reitz <mreitz@redhat.com>

Date: Tue, 23 Jul 2019 14:45:46 +0100

Subject: [PATCH 08/14] nbd/server: Advertise actual minimum block size

RH-Author: Max Reitz <mreitz@redhat.com>

Message-id: <20190723144546.23701-8-mreitz@redhat.com>

Patchwork-id: 89652

O-Subject: [RHEL-8.1.0 qemu-kvm PATCH 7/7] nbd/server: Advertise actual minimum block size

Bugzilla: 1678979

RH-Acked-by: Kevin Wolf <kwolf@redhat.com>

RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>

RH-Acked-by: John Snow <jsnow@redhat.com>

From: Eric Blake <eblake@redhat.com>

Both NBD_CMD_BLOCK_STATUS and structured NBD_CMD_READ will split their

reply according to bdrv_block_status() boundaries. If the block device

has a request_alignment smaller than 512, but we advertise a block

alignment of 512 to the client, then this can result in the server

reply violating client expectations by reporting a smaller region of

the export than what the client is permitted to address (although this

is less of an issue for qemu 4.0 clients, given recent client patches

to overlook our non-compliance at EOF).  Since it's always better to

be strict in what we send, it is worth advertising the actual minimum

block limit rather than blindly rounding it up to 512.

Note that this patch is not foolproof - it is still possible to

provoke non-compliant server behavior using:

$ qemu-nbd --image-opts driver=blkdebug,align=512,image.driver=file,image.filename=/path/to/non-aligned-file

That is arguably a bug in the blkdebug driver (it should never pass

back block status smaller than its alignment, even if it has to make

multiple bdrv_get_status calls and determine the

least-common-denominator status among the group to return). It may

also be possible to observe issues with a backing layer with smaller

alignment than the active layer, although so far I have been unable to

write a reliable iotest for that scenario (but again, an issue like

that could be argued to be a bug in the block layer, or something

where we need a flag to bdrv_block_status() to state whether the

result must be aligned to the current layer's limits or can be

subdivided for accuracy when chasing backing files).

Anyways, as blkdebug is not normally used, and as this patch makes our

server more interoperable with qemu 3.1 clients, it is worth applying

now, even while we still work on a larger patch series for the 4.1

timeframe to have byte-accurate file lengths.

Note that the iotests output changes - for 223 and 233, we can see the

server's better granularity advertisement; and for 241, the three test

cases have the following effects:

- natural alignment: the server's smaller alignment is now advertised,

and the hole reported at EOF is now the right result; we've gotten rid

of the server's non-compliance

- forced server alignment: the server still advertises 512 bytes, but

still sends a mid-sector hole. This is still a server compliance bug,

which needs to be fixed in the block layer in a later patch; output

does not change because the client is already being tolerant of the

non-compliance

- forced client alignment: the server's smaller alignment means that

the client now sees the server's status change mid-sector without any

protocol violations, but the fact that the map shows an unaligned

mid-sector hole is evidence of the block layer problems with aligned

block status, to be fixed in a later patch

Signed-off-by: Eric Blake <eblake@redhat.com>

Message-Id: <20190329042750.14704-7-eblake@redhat.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

[eblake: rebase to enhanced iotest 241 coverage]

(cherry picked from commit b0245d6478ea5906e3d7a542244d5c015fd47bc7)

Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

Conflicts:

- tests/qemu-iotests/223.out: We are missing

  ddd09448fd833d646952c769ae9ce3d39bee989f downstream, which adds

  qemu-nbd --list tests to 223.  (qemu-nbd --list does not exist

  downstream.)

- tests/qemu-iotests/233.out: Does not exist downstream.

- tests/qemu-iotests/241.out: Does not exist downstream, because it

  would require qemu-nbd --list.

Signed-off-by: Max Reitz <mreitz@redhat.com>

Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>

---

 nbd/server.c | 13 ++++++++-----

 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c

index e094300..96b6631 100644

--- a/nbd/server.c

+++ b/nbd/server.c

@@ -608,13 +608,16 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags,

     /* Send NBD_INFO_BLOCK_SIZE always, but tweak the minimum size

      * according to whether the client requested it, and according to

      * whether this is OPT_INFO or OPT_GO. */

-    /* minimum - 1 for back-compat, or 512 if client is new enough.

-     * TODO: consult blk_bs(blk)->bl.request_alignment? */

-    sizes[0] =

-            (client->opt == NBD_OPT_INFO || blocksize) ? BDRV_SECTOR_SIZE : 1;

+    /* minimum - 1 for back-compat, or actual if client will obey it. */

+    if (client->opt == NBD_OPT_INFO || blocksize) {

+        sizes[0] = blk_get_request_alignment(exp->blk);

+    } else {

+        sizes[0] = 1;

+    }

+    assert(sizes[0] <= NBD_MAX_BUFFER_SIZE);

     /* preferred - Hard-code to 4096 for now.

      * TODO: is blk_bs(blk)->bl.opt_transfer appropriate? */

-    sizes[1] = 4096;

+    sizes[1] = MAX(4096, sizes[0]);

     /* maximum - At most 32M, but smaller as appropriate. */

     sizes[2] = MIN(blk_get_max_transfer(exp->blk), NBD_MAX_BUFFER_SIZE);

     trace_nbd_negotiate_handle_info_block_size(sizes[0], sizes[1], sizes[2]);

-- 

1.8.3.1