[PATCH 3/3] file-posix: fix max_iov for /dev/sg devices

[Paolo Bonzini]

Even though it was only called for devices that have bs->sg set (which
must be character devices),
sg_get_max_segments looked at /sys/dev/block which only works for
block devices.

On Linux the sg driver has its own way to provide the maximum number of
iovecs in a scatter/gather list.
---
 block/file-posix.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/file-posix.c b/block/file-posix.c
index e37a6bb8de..b348b37ccb 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1198,6 +1198,17 @@ static int sg_get_max_segments(int fd, struct stat *st)
     int sysfd = -1;
     long max_segments;
 
+    if (S_ISCHR(st->st_mode)) {
+        if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
+            return ret;
+        }
+        return -EIO;
+    }
+
+    if (!S_ISBLK(st->st_mode)) {
+        return -ENOTSUP;
+    }
+
     sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
                                 major(st->st_rdev), minor(st->st_rdev));
     sysfd = open(sysfspath, O_RDONLY);
-- 
2.30.1