[PATCH v2 1/2] block/file-posix.c: Use pwritev2() with RWF_DSYNC for FUA

[Pinku Deb Nath <prantoran@gmail.com>]

Full Unit Access (FUA) is an optimization where a disk write with the
flag set will be persisted to disk immediately instead of potentially
remaining in the disk's write cache. This commit address the todo task
for using pwritev2() with RWF_DSYNC in the thread pool section of
raw_co_prw(), if pwritev2 with RWF_DSYNC is available in the host,
which is alway for Linux kernel >= 4.7. The intent for FUA is indicated
with the BDRV_REQ_FUA flag. The old code paths are preserved in case
BDRV_REQ_FUA is off or pwritev2() with RWF_DSYNC is not available.

During testing, I observed that the BDRV_REQ_FUA is always turned on
when blk->enable_write_cache is not set in block/block-backend.c, so
I commented this section off during testing:
https://gitlab.com/qemu-project/qemu/-/blob/master/block/block-backend.c?ref_type=heads#L1432-1434

Signed-off-by: Pinku Deb Nath <prantoran@gmail.com>
---
 block/file-posix.c | 42 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 34 insertions(+), 8 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 56d1972d15..34de816eab 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -229,6 +229,7 @@ typedef struct RawPosixAIOData {
             unsigned long op;
         } zone_mgmt;
     };
+    BdrvRequestFlags flags;
 } RawPosixAIOData;
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
@@ -1674,6 +1675,16 @@ qemu_pwritev(int fd, const struct iovec *iov, int nr_iov, off_t offset)
     return pwritev(fd, iov, nr_iov, offset);
 }
 
+static ssize_t
+qemu_pwrite_fua(int fd, const struct iovec *iov, int nr_iov, off_t offset)
+{
+#ifdef RWF_DSYNC
+    return pwritev2(fd, iov, nr_iov, offset, RWF_DSYNC);
+#else
+    return pwritev2(fd, iov, nr_iov, offset, 0);
+#endif
+}
+
 #else
 
 static bool preadv_present = false;
@@ -1698,10 +1709,15 @@ static ssize_t handle_aiocb_rw_vector(RawPosixAIOData *aiocb)
 
     len = RETRY_ON_EINTR(
         (aiocb->aio_type & (QEMU_AIO_WRITE | QEMU_AIO_ZONE_APPEND)) ?
-            qemu_pwritev(aiocb->aio_fildes,
-                           aiocb->io.iov,
-                           aiocb->io.niov,
-                           aiocb->aio_offset) :
+            (aiocb->flags &  BDRV_REQ_FUA) ?
+                qemu_pwrite_fua(aiocb->aio_fildes,
+                                aiocb->io.iov,
+                                aiocb->io.niov,
+                                aiocb->aio_offset) :
+                qemu_pwritev(aiocb->aio_fildes,
+                            aiocb->io.iov,
+                            aiocb->io.niov,
+                            aiocb->aio_offset) :
             qemu_preadv(aiocb->aio_fildes,
                           aiocb->io.iov,
                           aiocb->io.niov,
@@ -1727,10 +1743,17 @@ static ssize_t handle_aiocb_rw_linear(RawPosixAIOData *aiocb, char *buf)
 
     while (offset < aiocb->aio_nbytes) {
         if (aiocb->aio_type & (QEMU_AIO_WRITE | QEMU_AIO_ZONE_APPEND)) {
-            len = pwrite(aiocb->aio_fildes,
-                         (const char *)buf + offset,
-                         aiocb->aio_nbytes - offset,
-                         aiocb->aio_offset + offset);
+            if (aiocb->flags & BDRV_REQ_FUA) {
+                len = qemu_pwrite_fua(aiocb->aio_fildes,
+                                    aiocb->io.iov,
+                                    aiocb->io.niov,
+                                    aiocb->aio_offset);
+            } else {
+                len = pwrite(aiocb->aio_fildes,
+                            (const char *)buf + offset,
+                            aiocb->aio_nbytes - offset,
+                            aiocb->aio_offset + offset);
+            }
         } else {
             len = pread(aiocb->aio_fildes,
                         buf + offset,
@@ -2539,14 +2562,17 @@ static int coroutine_fn raw_co_prw(BlockDriverState *bs, int64_t *offset_ptr,
             .iov            = qiov->iov,
             .niov           = qiov->niov,
         },
+        .flags          = flags,
     };
 
     assert(qiov->size == bytes);
     ret = raw_thread_pool_submit(handle_aiocb_rw, &acb);
+#ifndef RWD_DSYNC
     if (ret == 0 && (flags & BDRV_REQ_FUA)) {
         /* TODO Use pwritev2() instead if it's available */
         ret = raw_co_flush_to_disk(bs);
     }
+#endif
     goto out; /* Avoid the compiler err of unused label */
 
 out:
-- 
2.43.0