block: Switch discard length bounds to byte-based

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 <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 11 insertions, 5 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 7a4a00fdfc..a3e69fd93a 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;
-
-    /* optimal alignment for discard requests in sectors */
-    int64_t discard_alignment;
+    /* 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 bytes, must be power
+     * of 2, less than max_pdiscard 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