[PATCH v4 10/12] virtio-blk: derive the maximum zone append size

[Niklas Cassel <cassel@kernel.org>]

The max_append_sectors field of virtio_blk_zoned_characteristics must be
set by the device to the largest zone append request that can be issued
to it, and a value of zero tells the guest driver that zone append is not
supported at all (virtio 1.4, 5.2.5.2). Linux refuses to attach a zoned
device that reports zero.

We pass BlockLimits.max_append_sectors straight through, which makes that
field mean "zone append unsupported" when it is unset, rather than "this
backend imposes no limit of its own". Only a backend that has a limit of
its own has anything to put there.

Derive the value instead. A backend limit is honoured when there is one,
and otherwise the request is bounded by the zone size, since an append
cannot cross a zone boundary, and by the largest request the block layer
can carry. The result cannot be zero.

A backend that carries out an append itself, rather than passing it to a
device that has a limit of its own, is the one that knows how large a
request its implementation can take, so it reports that in
BlockLimits.max_append_sectors and this does not have to guess at it.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
 hw/block/virtio-blk.c | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index c2acf59c6d..131516a167 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -525,6 +525,27 @@ typedef struct ZoneCmdData {
     };
 } ZoneCmdData;
 
+/*
+ * The maximum zone append data size that the device reports to the driver in
+ * virtio_blk_zoned_characteristics, in 512 byte sectors.
+ *
+ * A backend that has no limit of its own leaves BlockLimits.max_append_sectors
+ * at zero, in which case the limit is whatever else bounds the request: an
+ * append cannot cross a zone boundary, and the block layer cannot carry a
+ * larger one. The result is never zero, which the driver would read as zone
+ * append not being supported at all.
+ */
+static uint32_t virtio_blk_max_append_sectors(VirtIOBlock *s)
+{
+    BlockDriverState *bs = blk_bs(s->blk);
+    uint64_t sectors;
+
+    sectors = MIN_NON_ZERO(bs->bl.zone_size >> BDRV_SECTOR_BITS,
+                           bs->bl.max_append_sectors);
+
+    return MIN_NON_ZERO(sectors, BDRV_REQUEST_MAX_SECTORS);
+}
+
 /*
  * check zoned_request: error checking before issuing requests. If all checks
  * passed, return true.
@@ -560,12 +581,8 @@ static bool check_zoned_request(VirtIOBlock *s, int64_t offset, int64_t len,
             return false;
         }
 
-        if (len / 512 > bs->bl.max_append_sectors) {
-            if (bs->bl.max_append_sectors == 0) {
-                *status = VIRTIO_BLK_S_UNSUPP;
-            } else {
-                *status = VIRTIO_BLK_S_ZONE_INVALID_CMD;
-            }
+        if ((len >> BDRV_SECTOR_BITS) > virtio_blk_max_append_sectors(s)) {
+            *status = VIRTIO_BLK_S_ZONE_INVALID_CMD;
             return false;
         }
     }
@@ -1313,7 +1330,7 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
         virtio_stl_p(vdev, &blkcfg.zoned.write_granularity,
                      blkconf_zone_write_granularity(conf));
         virtio_stl_p(vdev, &blkcfg.zoned.max_append_sectors,
-                     bs->bl.max_append_sectors);
+                     virtio_blk_max_append_sectors(s));
     } else {
         blkcfg.zoned.model = VIRTIO_BLK_Z_NONE;
     }
-- 
2.55.0