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Date: Tue, 27 Feb 2018 17:34:30 +0800
From: Fam Zheng <famz@redhat.com>
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References: <20180122220806.22154-1-mreitz@redhat.com>
	<20180122220806.22154-15-mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 14/16] block/mirror: Add active mirroring
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To: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, John Snow <jsnow@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

On Mon, 01/22 23:08, Max Reitz wrote:
> @@ -1151,7 +1285,48 @@ static int coroutine_fn bdrv_mirror_top_preadv(BlockDriverState *bs,
>  static int coroutine_fn bdrv_mirror_top_pwritev(BlockDriverState *bs,
>      uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags)
>  {
> -    return bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
> +    MirrorOp *op = NULL;
> +    MirrorBDSOpaque *s = bs->opaque;
> +    QEMUIOVector bounce_qiov;
> +    void *bounce_buf;
> +    int ret = 0;
> +    bool copy_to_target;
> +
> +    copy_to_target = s->job->ret >= 0 &&
> +                     s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
> +
> +    if (copy_to_target) {
> +        /* The guest might concurrently modify the data to write; but
> +         * the data on source and destination must match, so we have
> +         * to use a bounce buffer if we are going to write to the
> +         * target now. */
> +        bounce_buf = qemu_blockalign(bs, bytes);
> +        iov_to_buf_full(qiov->iov, qiov->niov, 0, bounce_buf, bytes);

Quorum doesn't use a bounce buffer, so I think we can get away without it too: a
guest concurrently modifying the buffer isn't a concern in practice.

> +
> +        qemu_iovec_init(&bounce_qiov, 1);
> +        qemu_iovec_add(&bounce_qiov, bounce_buf, bytes);
> +        qiov = &bounce_qiov;
> +
> +        op = active_write_prepare(s->job, offset, bytes);
> +    }
> +
> +    ret = bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
> +    if (ret < 0) {
> +        goto out;
> +    }
> +
> +    if (copy_to_target) {
> +        do_sync_target_write(s->job, offset, bytes, qiov, flags);
> +    }
> +
> +out:
> +    if (copy_to_target) {
> +        active_write_settle(op);
> +
> +        qemu_iovec_destroy(&bounce_qiov);
> +        qemu_vfree(bounce_buf);
> +    }
> +    return ret;
>  }
>  
>  static int coroutine_fn bdrv_mirror_top_flush(BlockDriverState *bs)

Don't you need to update bdrv_mirror_top_pdiscard and bdrv_mirror_top_pwritev
too?

Fam