[PATCH v4 1/7] file-posix: fix max_iov for /dev/sg devices

[Paolo Bonzini <pbonzini@redhat.com>]

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, so add support for it.  The block device
path is kept because it will be reinstated in the next patches.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/file-posix.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/file-posix.c b/block/file-posix.c
index f37dfc10b3..536998a1d6 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1180,6 +1180,17 @@ static int sg_get_max_segments(int fd)
         goto out;
     }
 
+    if (S_ISCHR(st.st_mode)) {
+        if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
+            return ret;
+        }
+        return -ENOTSUP;
+    }
+
+    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);
-- 
2.31.1