Le Friday 17 Jan 2014 à 15:15:10 (+0100), Kevin Wolf a écrit :
> This patch changes bdrv_co_do_pwritev() to actually be what its name
> promises. If requests aren't properly aligned, it performs a RMW.
>
> Requests touching the same block are serialised against the RMW request.
> Further optimisation of this is possible by differentiating types of
> requests (concurrent reads should actually be okay here).
>
> Signed-off-by: Kevin Wolf <kw...@redhat.com>
> Reviewed-by: Max Reitz <mre...@redhat.com>
> ---
> block.c | 86
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 85 insertions(+), 1 deletion(-)
>
> diff --git a/block.c b/block.c
> index 55e8c69..859e1aa 100644
> --- a/block.c
> +++ b/block.c
> @@ -3055,6 +3055,12 @@ static int coroutine_fn
> bdrv_co_do_pwritev(BlockDriverState *bs,
> BdrvRequestFlags flags)
> {
> BdrvTrackedRequest req;
> + /* TODO Lift BDRV_SECTOR_SIZE restriction in BlockDriver interface */
> + uint64_t align = MAX(BDRV_SECTOR_SIZE, bs->request_alignment);
> + uint8_t *head_buf = NULL;
> + uint8_t *tail_buf = NULL;
> + QEMUIOVector local_qiov;
> + bool use_local_qiov = false;
> int ret;
>
> if (!bs->drv) {
> @@ -3073,10 +3079,88 @@ static int coroutine_fn
> bdrv_co_do_pwritev(BlockDriverState *bs,
> bdrv_io_limits_intercept(bs, bytes >> BDRV_SECTOR_BITS, true);
> }
>
> + /*
> + * Align write if necessary by performing a read-modify-write cycle.
> + * Pad qiov with the read parts and be sure to have a tracked request not
> + * only for bdrv_aligned_pwritev, but also for the reads of the RMW
> cycle.
> + */
> tracked_request_begin(&req, bs, offset, bytes, true);
> - ret = bdrv_aligned_pwritev(bs, &req, offset, bytes, qiov, flags);
> +
> + if (offset & (align - 1)) {
> + QEMUIOVector head_qiov;
> + struct iovec head_iov;
> +
> + mark_request_serialising(&req, align);
> + wait_serialising_requests(&req);
> +
> + head_buf = qemu_blockalign(bs, align);
> + head_iov = (struct iovec) {
> + .iov_base = head_buf,
> + .iov_len = align,
> + };
> + qemu_iovec_init_external(&head_qiov, &head_iov, 1);
> +
> + ret = bdrv_aligned_preadv(bs, &req, offset & ~(align - 1), align,
> + align, &head_qiov, 0);
> + if (ret < 0) {
> + goto fail;
> + }
> +
> + qemu_iovec_init(&local_qiov, qiov->niov + 2);
> + qemu_iovec_add(&local_qiov, head_buf, offset & (align - 1));
> + qemu_iovec_concat(&local_qiov, qiov, 0, qiov->size);
> + use_local_qiov = true;
> +
> + bytes += offset & (align - 1);
> + offset = offset & ~(align - 1);
> + }
> +
> + if ((offset + bytes) & (align - 1)) {
> + QEMUIOVector tail_qiov;
> + struct iovec tail_iov;
> + size_t tail_bytes;
> +
> + mark_request_serialising(&req, align);
> + wait_serialising_requests(&req);
> +
> + tail_buf = qemu_blockalign(bs, align);
> + tail_iov = (struct iovec) {
> + .iov_base = tail_buf,
> + .iov_len = align,
> + };
> + qemu_iovec_init_external(&tail_qiov, &tail_iov, 1);
> +
> + ret = bdrv_aligned_preadv(bs, &req, (offset + bytes) & ~(align - 1),
> align,
> + align, &tail_qiov, 0);
> + if (ret < 0) {
> + goto fail;
> + }
> +
> + if (!use_local_qiov) {
> + qemu_iovec_init(&local_qiov, qiov->niov + 1);
> + qemu_iovec_concat(&local_qiov, qiov, 0, qiov->size);
> + use_local_qiov = true;
> + }
> +
> + tail_bytes = (offset + bytes) & (align - 1);
> + qemu_iovec_add(&local_qiov, tail_buf + tail_bytes, align -
> tail_bytes);
> +
> + bytes = ROUND_UP(bytes, align);
> + }
> +
> + ret = bdrv_aligned_pwritev(bs, &req, offset, bytes,
> + use_local_qiov ? &local_qiov : qiov,
> + flags);
> +
> +fail:
> tracked_request_end(&req);
>
> + if (use_local_qiov) {
> + qemu_iovec_destroy(&local_qiov);
> + qemu_vfree(head_buf);
> + qemu_vfree(tail_buf);
> + }
> +
> return ret;
> }
>
> --
> 1.8.1.4
>
>
Reviewed-by: Benoit Canet <ben...@irqsave.net>
