[PATCH v4 2/2] file-posix: add sg_get_max_segments that actually works with sg

[Tom Yan <tom.ty89@gmail.com>]

sg devices have different major/minor than their corresponding
block devices. Using sysfs to get max segments never really worked
for them.

Fortunately the sg driver provides an ioctl to get sg_tablesize,
which is apparently equivalent to max segments.

Signed-off-by: Tom Yan <tom.ty89@gmail.com>
---
 block/file-posix.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 411a23cf99..9e37594145 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1178,6 +1178,21 @@ static int sg_get_max_transfer_length(int fd)
 #endif
 }
 
+static int sg_get_max_segments(int fd)
+{
+#ifdef SG_GET_SG_TABLESIZE
+    long max_segments = 0;
+
+    if (ioctl(fd, SG_GET_SG_TABLESIZE, &max_segments) == 0) {
+        return max_segments;
+    } else {
+        return -errno;
+    }
+#else
+    return -ENOSYS;
+#endif
+}
+
 static int get_max_transfer_length(int fd)
 {
 #if defined(BLKSECTGET) && defined(BLKSSZGET)
@@ -1268,7 +1283,7 @@ static void hdev_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.max_transfer = pow2floor(ret);
     }
 
-    ret = get_max_segments(s->fd);
+    ret = bs->sg ? sg_get_max_segments(s->fd) : get_max_segments(s->fd);
     if (ret > 0) {
         bs->bl.max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
                                            ret * qemu_real_host_page_size);
-- 
2.28.0