[Qemu-devel] [PATCH 4/4] nbd/server: Support requests of additional block sizing info

[Eric Blake <eblake@redhat.com>]

The NBD spec is clarifying [1] that a server may want to advertise
different limits for READ/WRITE (in our case, 32M) than for
TRIM/ZERO (in our case, nearly 4G).  Implement the server
side support for these alternate limits, by always advertising
the new information (a compliant client must ignore a gratuitous
advertisement if it is unknown).

[1] https://lists.debian.org/nbd/2018/03/msg00048.html

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

---

The given URL for the NBD spec was v3; it will change to be a v4
version of that patch in part to point back to this qemu commit
as a proof of implementation.
---
 nbd/server.c     | 30 ++++++++++++++++++++++++++++++
 nbd/trace-events |  2 ++
 2 files changed, 32 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index 9e1f2271784..39502b58446 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -617,6 +617,36 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags,
         return rc;
     }

+    /* Send NBD_INFO_TRIM_SIZE always. */
+    /* minimum - Hard-code to 4096 for now, matching preferred block.
+     * TODO: consult blk_bs(blk)->bl.pdiscard_alignment? */
+    sizes[0] = 4096;
+    /* maximum - < 2G (then block layer fragments to bl.max_pdiscard). */
+    sizes[1] = QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES, sizes[0]);
+    trace_nbd_negotiate_handle_info_trim_size(sizes[0], sizes[1]);
+    cpu_to_be32s(&sizes[0]);
+    cpu_to_be32s(&sizes[1]);
+    rc = nbd_negotiate_send_info(client, NBD_INFO_TRIM_SIZE,
+                                 2 * sizeof(sizes[0]), sizes, errp);
+    if (rc < 0) {
+        return rc;
+    }
+
+    /* Send NBD_INFO_ZERO_SIZE always. */
+    /* minimum - Hard-code to 4096 for now, matching preferred block.
+     * TODO: consult blk_bs(blk)->bl.pwrite_zeroes_alignment? */
+    sizes[0] = 4096;
+    /* maximum - < 2G (then block layer fragments to bl.max_pwrite_zeroes). */
+    sizes[1] = QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES, sizes[0]);
+    trace_nbd_negotiate_handle_info_zero_size(sizes[0], sizes[1]);
+    cpu_to_be32s(&sizes[0]);
+    cpu_to_be32s(&sizes[1]);
+    rc = nbd_negotiate_send_info(client, NBD_INFO_ZERO_SIZE,
+                                 2 * sizeof(sizes[0]), sizes, errp);
+    if (rc < 0) {
+        return rc;
+    }
+
     /* Send NBD_INFO_EXPORT always */
     trace_nbd_negotiate_new_style_size_flags(exp->size,
                                              exp->nbdflags | myflags);
diff --git a/nbd/trace-events b/nbd/trace-events
index ddb9d0a2b3e..d8849c43b21 100644
--- a/nbd/trace-events
+++ b/nbd/trace-events
@@ -46,6 +46,8 @@ nbd_negotiate_send_info(int info, const char *name, uint32_t length) "Sending NB
 nbd_negotiate_handle_info_requests(int requests) "Client requested %d items of info"
 nbd_negotiate_handle_info_request(int request, const char *name) "Client requested info %d (%s)"
 nbd_negotiate_handle_info_block_size(uint32_t minimum, uint32_t preferred, uint32_t maximum) "advertising minimum 0x%" PRIx32 ", preferred 0x%" PRIx32 ", maximum 0x%" PRIx32
+nbd_negotiate_handle_info_trim_size(uint32_t minimum, uint32_t maximum) "advertising minimum 0x%" PRIx32 ", maximum 0x%" PRIx32
+nbd_negotiate_handle_info_zero_size(uint32_t minimum, uint32_t maximum) "advertising minimum 0x%" PRIx32 ", maximum 0x%" PRIx32
 nbd_negotiate_handle_starttls(void) "Setting up TLS"
 nbd_negotiate_handle_starttls_handshake(void) "Starting TLS handshake"
 nbd_negotiate_meta_context(const char *optname, const char *export, uint32_t queries) "Client requested %s for export %s, with %" PRIu32 " queries"
-- 
2.14.3