[Qemu-devel] [PATCH 1/2] raw-posix: Fetch max sectors for host block device from sysfs

[Fam Zheng]

This is sometimes a useful value we should count in.

Signed-off-by: Fam Zheng <address@hidden>
---
 block/raw-posix.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/block/raw-posix.c b/block/raw-posix.c
index a4f5a1b..d3796ad 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -729,9 +729,56 @@ static void raw_reopen_abort(BDRVReopenState *state)
     state->opaque = NULL;
 }
 
+static int hdev_get_max_transfer_length(dev_t dev)
+{
+    int ret;
+    int fd;
+    char *path;
+    const char *end;
+    char buf[32];
+    long len;
+
+    path = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_sectors_kb",
+                           major(dev), minor(dev));
+    fd = open(path, O_RDONLY);
+    if (fd < 0) {
+        ret = -errno;
+        goto out;
+    }
+    ret = read(fd, buf, sizeof(buf));
+    if (ret < 0) {
+        ret = -errno;
+        goto out;
+    } else if (ret == 0) {
+        ret = -EIO;
+        goto out;
+    }
+    buf[ret] = 0;
+    /* The file is ended with '\n', pass 'end' to accept that. */
+    ret = qemu_strtol(buf, &end, 10, &len);
+    if (ret == 0 && end && *end == '\n') {
+        ret = len * 1024 / BDRV_SECTOR_SIZE;
+    }
+
+    close(fd);
+out:
+    g_free(path);
+    return ret;
+}
+
 static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     BDRVRawState *s = bs->opaque;
+    struct stat st;
+
+    if (!fstat(s->fd, &st)) {
+        if (S_ISBLK(st.st_mode)) {
+            int ret = hdev_get_max_transfer_length(st.st_rdev);
+            if (ret >= 0) {
+                bs->bl.max_transfer_length = ret;
+            }
+        }
+    }
 
     raw_probe_alignment(bs, s->fd, errp);
     bs->bl.min_mem_alignment = s->buf_align;
-- 
2.8.2