[Qemu-devel] [PATCH 1/1] raw-posix.c - use max transfer length / max segemnt count only for SCSI passthrough

[Maxim Levitsky]

Regular block devices (/dev/sda*, /dev/nvme*, etc) interface is not limited
by the underlying storage limits, but rather the kernel block layer
takes care to split the requests that are too large/fragmented.

Doing so allows us to have less overhead in qemu.

Signed-off-by: Maxim Levitsky <address@hidden>
---
 block/file-posix.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index ab05b51a66..66dad34f8a 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1038,15 +1038,13 @@ static void raw_reopen_abort(BDRVReopenState *state)
     s->reopen_state = NULL;
 }
 
-static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
+static int sg_get_max_transfer_length(BlockDriverState *bs, int fd)
 {
 #ifdef BLKSECTGET
     int max_bytes = 0;
-    short max_sectors = 0;
-    if (bs->sg && ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
+
+    if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
         return max_bytes;
-    } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
-        return max_sectors << BDRV_SECTOR_BITS;
     } else {
         return -errno;
     }
@@ -1055,7 +1053,7 @@ static int hdev_get_max_transfer_length(BlockDriverState 
*bs, int fd)
 #endif
 }
 
-static int hdev_get_max_segments(const struct stat *st)
+static int sg_get_max_segments(const struct stat *st)
 {
 #ifdef CONFIG_LINUX
     char buf[32];
@@ -1106,12 +1104,12 @@ static void raw_refresh_limits(BlockDriverState *bs, 
Error **errp)
     struct stat st;
 
     if (!fstat(s->fd, &st)) {
-        if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) {
-            int ret = hdev_get_max_transfer_length(bs, s->fd);
+        if (bs->sg) {
+            int ret = sg_get_max_transfer_length(bs, s->fd);
             if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
                 bs->bl.max_transfer = pow2floor(ret);
             }
-            ret = hdev_get_max_segments(&st);
+            ret = sg_get_max_segments(&st);
             if (ret > 0) {
                 bs->bl.max_transfer = MIN(bs->bl.max_transfer,
                                           ret * getpagesize());
-- 
2.17.2