[Qemu-devel] [RFC PATCH 03/19] block: Don't use guest sector size for qemu_blockalign()

[Kevin Wolf <kwolf@redhat.com>]

bs->buffer_alignment is set by the device emulation and contains the
logical block size of the guest device. This isn't something that the
block layer should know, and even less something to use for determining
the right alignment of buffers to be used for the host.

The new function bdrv_opt_mem_align() allows for hooks in a BlockDriver
so that it can tell the qemu block layer the optimal alignment to be
used so that no bounce buffer must be used in the driver.

This patch may change the buffer alignment from 4k to 512 for all
callers that used qemu_blockalign() with the top-level image format
BlockDriverState. The value was never propagated to other levels in the
tree, so in particular raw-posix never required anything else than 512.

While on disks with 4k sectors direct I/O requires a 4k alignment,
memory may still be okay when aligned to 512 byte boundaries. This is
what must have happened in practice, because otherwise this would
already have failed earlier. Therefore I don't expect regressions even
with this intermediate state. Later, raw-posix can implement the hook
and expose a different memory alignment requirement.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c                   | 32 +++++++++++++++++++++++++++++---
 include/block/block.h     |  1 +
 include/block/block_int.h |  4 ++++
 3 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 613201b..669793b 100644
--- a/block.c
+++ b/block.c
@@ -213,6 +213,31 @@ static void bdrv_io_limits_intercept(BlockDriverState *bs,
     qemu_co_queue_next(&bs->throttled_reqs[is_write]);
 }
 
+size_t bdrv_opt_mem_align(BlockDriverState *bs)
+{
+    size_t alignment;
+
+    if (!bs || !bs->drv) {
+        /* 4k should be on the safe side */
+        return 4096;
+    }
+
+    if (bs->drv->bdrv_opt_mem_align) {
+        return bs->drv->bdrv_opt_mem_align(bs);
+    }
+
+    if (bs->file) {
+        alignment = bdrv_opt_mem_align(bs->file);
+    } else {
+        alignment = 512;
+    }
+
+    if (bs->backing_hd) {
+        alignment = MAX(alignment, bdrv_opt_mem_align(bs->backing_hd));
+    }
+    return alignment;
+}
+
 /* check if the path starts with "<protocol>:" */
 static int path_has_protocol(const char *path)
 {
@@ -4335,7 +4360,7 @@ void bdrv_set_buffer_alignment(BlockDriverState *bs, int align)
 
 void *qemu_blockalign(BlockDriverState *bs, size_t size)
 {
-    return qemu_memalign((bs && bs->buffer_alignment) ? bs->buffer_alignment : 512, size);
+    return qemu_memalign(bdrv_opt_mem_align(bs), size);
 }
 
 /*
@@ -4344,12 +4369,13 @@ void *qemu_blockalign(BlockDriverState *bs, size_t size)
 bool bdrv_qiov_is_aligned(BlockDriverState *bs, QEMUIOVector *qiov)
 {
     int i;
+    size_t alignment = bdrv_opt_mem_align(bs);
 
     for (i = 0; i < qiov->niov; i++) {
-        if ((uintptr_t) qiov->iov[i].iov_base % bs->buffer_alignment) {
+        if ((uintptr_t) qiov->iov[i].iov_base % alignment) {
             return false;
         }
-        if (qiov->iov[i].iov_len % bs->buffer_alignment) {
+        if (qiov->iov[i].iov_len % alignment) {
             return false;
         }
     }
diff --git a/include/block/block.h b/include/block/block.h
index 3560deb..d262c0e 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -383,6 +383,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
                      char *options, uint64_t img_size, int flags,
                      Error **errp, bool quiet);
 
+size_t bdrv_opt_mem_align(BlockDriverState *bs);
 void bdrv_set_buffer_alignment(BlockDriverState *bs, int align);
 void *qemu_blockalign(BlockDriverState *bs, size_t size);
 bool bdrv_qiov_is_aligned(BlockDriverState *bs, QEMUIOVector *qiov);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 1666066..6a84f83 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -119,6 +119,10 @@ struct BlockDriver {
         int64_t sector_num, int nb_sectors,
         BlockDriverCompletionFunc *cb, void *opaque);
 
+    /* Returns the alignment in bytes that is required so that no bounce buffer
+     * is required throughout the stack */
+    int (*bdrv_opt_mem_align)(BlockDriverState *bs);
+
     int coroutine_fn (*bdrv_co_readv)(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);
     int coroutine_fn (*bdrv_co_writev)(BlockDriverState *bs,
-- 
1.8.1.4