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

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013/12/7 1:22, Kevin Wolf 写道:
> 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));
> +    }

  Maybe I didn't understand the commit message correctly, does this code
intend to get MAX alignment value in a chain? For example:
base(4096)->mid(512)->top(1024) results: 4096
The condition to traver the backing files seems complex.

> +    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,
>