[Qemu-devel] [PATCH v3 17/22] block: Switch discard length bounds to byte-based

[Eric Blake]

Sector-based limits are awkward to think about; in our on-going
quest to move to byte-based interfaces, convert max_discard and
discard_alignment.  Rename them, using 'pdiscard' as an aid to
track which remaining discard interfaces need conversion, and so
that the compiler will help us catch the change in semantics
across any rebased code.  The BlockLimits type is now completely
byte-based; and in iscsi.c, sector_limits_lun2qemu() is no
longer needed.

pdiscard_alignment is made unsigned (we use power-of-2 alignments
as bitmasks, where unsigned is easier to think about) while
leaving max_pdiscard signed (since we still have an 'int'
interface); this is comparable to what commit cf081fc did for
write zeroes limits.  We may later want to make everything an
unsigned 64-bit limit - but that requires a bigger code audit.

Signed-off-by: Eric Blake <address@hidden>

---
v3: split out write_zeroes wording tweaks, improve commit message
v2: rebase nbd and iscsi limits across earlier improvements
---
 include/block/block_int.h | 14 ++++++++++----
 block/io.c                | 16 +++++++++-------
 block/iscsi.c             | 19 ++++++-------------
 block/nbd.c               |  2 +-
 qemu-img.c                |  3 ++-
 5 files changed, 28 insertions(+), 26 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 7a4a00f..388ef80 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -324,11 +324,17 @@ struct BlockDriver {
 };

 typedef struct BlockLimits {
-    /* maximum number of sectors that can be discarded at once */
-    int max_discard;
+    /* maximum number of bytes that can be discarded at once (since it
+     * is signed, it must be < 2G, if set), should be multiple of
+     * pdiscard_alignment, but need not be power of 2. May be 0 if no
+     * inherent 32-bit limit */
+    int32_t max_pdiscard;

-    /* optimal alignment for discard requests in sectors */
-    int64_t discard_alignment;
+    /* optimal alignment for discard requests in bytes, must be power
+     * of 2, less than max_discard if that is set, and multiple of
+     * bs->request_alignment. May be 0 if bs->request_alignment is
+     * good enough */
+    uint32_t pdiscard_alignment;

     /* maximum number of bytes that can zeroized at once (since it is
      * signed, it must be < 2G, if set), should be multiple of
diff --git a/block/io.c b/block/io.c
index 8ca9d43..0f15d05 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2368,19 +2368,21 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, 
int64_t sector_num,
         goto out;
     }

-    max_discard = MIN_NON_ZERO(bs->bl.max_discard, BDRV_REQUEST_MAX_SECTORS);
+    max_discard = MIN_NON_ZERO(bs->bl.max_pdiscard >> BDRV_SECTOR_BITS,
+                               BDRV_REQUEST_MAX_SECTORS);
     while (nb_sectors > 0) {
         int ret;
         int num = nb_sectors;
+        int discard_alignment = bs->bl.pdiscard_alignment >> BDRV_SECTOR_BITS;

         /* align request */
-        if (bs->bl.discard_alignment &&
-            num >= bs->bl.discard_alignment &&
-            sector_num % bs->bl.discard_alignment) {
-            if (num > bs->bl.discard_alignment) {
-                num = bs->bl.discard_alignment;
+        if (discard_alignment &&
+            num >= discard_alignment &&
+            sector_num % discard_alignment) {
+            if (num > discard_alignment) {
+                num = discard_alignment;
             }
-            num -= sector_num % bs->bl.discard_alignment;
+            num -= sector_num % discard_alignment;
         }

         /* limit request size */
diff --git a/block/iscsi.c b/block/iscsi.c
index 368687d..0d16c31 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1696,13 +1696,6 @@ static void iscsi_close(BlockDriverState *bs)
     memset(iscsilun, 0, sizeof(IscsiLun));
 }

-static int sector_limits_lun2qemu(int64_t sector, IscsiLun *iscsilun)
-{
-    int limit = MIN(sector_lun2qemu(sector, iscsilun), INT_MAX / 2 + 1);
-
-    return limit < BDRV_REQUEST_MAX_SECTORS ? limit : 0;
-}
-
 static void iscsi_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     /* We don't actually refresh here, but just return data queried in
@@ -1722,14 +1715,14 @@ static void iscsi_refresh_limits(BlockDriverState *bs, 
Error **errp)
     }

     if (iscsilun->lbp.lbpu) {
-        if (iscsilun->bl.max_unmap < 0xffffffff) {
-            bs->bl.max_discard =
-                sector_limits_lun2qemu(iscsilun->bl.max_unmap, iscsilun);
+        if (iscsilun->bl.max_unmap < 0xffffffff / iscsilun->block_size) {
+            bs->bl.max_pdiscard =
+                iscsilun->bl.max_unmap * iscsilun->block_size;
         }
-        bs->bl.discard_alignment =
-            sector_limits_lun2qemu(iscsilun->bl.opt_unmap_gran, iscsilun);
+        bs->bl.pdiscard_alignment =
+            iscsilun->bl.opt_unmap_gran * iscsilun->block_size;
     } else {
-        bs->bl.discard_alignment = iscsilun->block_size >> BDRV_SECTOR_BITS;
+        bs->bl.pdiscard_alignment = iscsilun->block_size;
     }

     if (iscsilun->bl.max_ws_len < 0xffffffff / iscsilun->block_size) {
diff --git a/block/nbd.c b/block/nbd.c
index f5511ea..08e5b67 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -362,7 +362,7 @@ static int nbd_co_flush(BlockDriverState *bs)

 static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
 {
-    bs->bl.max_discard = NBD_MAX_SECTORS;
+    bs->bl.max_pdiscard = NBD_MAX_BUFFER_SIZE;
     bs->bl.max_transfer = NBD_MAX_BUFFER_SIZE;
 }

diff --git a/qemu-img.c b/qemu-img.c
index cf9876d..1237d61 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2084,7 +2084,8 @@ static int img_convert(int argc, char **argv)
     bufsectors = MIN(32768,
                      MAX(bufsectors,
                          MAX(out_bs->bl.opt_transfer >> BDRV_SECTOR_BITS,
-                             out_bs->bl.discard_alignment)));
+                             out_bs->bl.pdiscard_alignment >>
+                             BDRV_SECTOR_BITS)));

     if (skip_create) {
         int64_t output_sectors = blk_nb_sectors(out_blk);
-- 
2.5.5