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Re: [Qemu-devel] [PATCH 02/11] block/io: add bdrv_co_write_compressed

From: Pavel Butsykin
Subject: Re: [Qemu-devel] [PATCH 02/11] block/io: add bdrv_co_write_compressed
Date: Tue, 28 Jun 2016 14:35:55 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 28.06.2016 14:09, Kevin Wolf wrote:
Am 31.05.2016 um 11:15 hat Denis V. Lunev geschrieben:
From: Pavel Butsykin <address@hidden>

This patch just adds the interface to the bdrv_co_pwritev_compressed,
which is currently not used but will be useful for safe implementation of the
bdrv_co_write_compressed callback in format drivers.

Signed-off-by: Pavel Butsykin <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
CC: Jeff Cody <address@hidden>
CC: Markus Armbruster <address@hidden>
CC: Eric Blake <address@hidden>
CC: John Snow <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
CC: Kevin Wolf <address@hidden>
  block/io.c                | 78 +++++++++++++++++++++++++++++++++++++++++++----
  include/block/block_int.h |  5 +++
  qemu-img.c                |  2 +-
  3 files changed, 78 insertions(+), 7 deletions(-)

diff --git a/block/io.c b/block/io.c
index c5bb6ae..54cd9a4 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1779,8 +1779,8 @@ int bdrv_is_allocated_above(BlockDriverState *top,
      return 0;

-int bdrv_pwrite_compressed(BlockDriverState *bs, int64_t offset,
-                           const void *buf, int count)
+int coroutine_fn bdrv_co_pwritev_compressed(BlockDriverState *bs,
+    int64_t offset, unsigned int bytes, QEMUIOVector *qiov)
      BlockDriver *drv = bs->drv;
      int ret;
@@ -1788,18 +1788,84 @@ int bdrv_pwrite_compressed(BlockDriverState *bs, 
int64_t offset,
      if (!drv) {
          return -ENOMEDIUM;
-    if (!drv->bdrv_write_compressed) {
+    if (!drv->bdrv_co_write_compressed) {
          return -ENOTSUP;
-    ret = bdrv_check_byte_request(bs, offset, count);
+    ret = bdrv_check_byte_request(bs, offset, bytes);
      if (ret < 0) {
          return ret;

+    assert(qemu_in_coroutine());
+    return drv->bdrv_co_write_compressed(bs, offset >> BDRV_SECTOR_BITS,
+                                         bytes >> BDRV_SECTOR_BITS, qiov);
+typedef struct BdrvWriteCompressedCo {
+    BlockDriverState *bs;
+    int64_t offset;
+    QEMUIOVector *qiov;
+    int ret;
+} BdrvWriteCompressedCo;

I think you could just reuse RwCo here.

I reused BlkRwCo, and it's only a temporary change.

And in fact, this made we wonder whether we can reuse more from the
normal I/O path. If compressed writes could be made just another REQ_*
flag and we went through bdrv_co_pwritev(), we would automatically get
the alignment code if we don't want to convert drivers to byte-based.
And also the existing coroutine wrapper code would be reused instead
implementing it once more.

I'm not completely sure how special compressed writes really are, so
reusing the normal I/O path may or may not work. It might be worth
investigating, though.

--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -207,6 +207,9 @@ struct BlockDriver {
      int (*bdrv_write_compressed)(BlockDriverState *bs, int64_t sector_num,
                                   const uint8_t *buf, int nb_sectors);

+    int coroutine_fn (*bdrv_co_write_compressed)(BlockDriverState *bs,
+        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);

This should be called bdrv_co_writev_compressed because it's a vectored



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