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Date: Thu, 10 May 2018 10:25:50 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 3/9] qcow2: Implement copy offloading
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To: Fam Zheng <famz@redhat.com>
Cc: qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Ronnie Sahlberg <ronniesahlberg@gmail.com>, qemu-block@nongnu.org, Kevin Wolf <kwolf@redhat.com>, Peter Lieven <pl@kamp.de>, Max Reitz <mreitz@redhat.com>


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On Wed, May 09, 2018 at 10:58:09PM +0800, Fam Zheng wrote:
> +static int qcow2_co_copy_range_from(BlockDriverState *bs,
> +                                    BdrvChild *src, uint64_t src_offset,
> +                                    BdrvChild *dst, uint64_t dst_offset,
> +                                    uint64_t bytes, BdrvRequestFlags flags)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    int offset_in_cluster;
> +    int ret;
> +    unsigned int cur_bytes; /* number of bytes in current iteration */
> +    uint64_t cluster_offset = 0;
> +    BdrvChild *child = NULL;
> +
> +    assert(!bs->encrypted);
> +    qemu_co_mutex_lock(&s->lock);
> +
> +    while (bytes != 0) {
> +
> +        /* prepare next request */
> +        cur_bytes = MIN(bytes, INT_MAX);
> +
> +        ret = qcow2_get_cluster_offset(bs, src_offset, &cur_bytes, &cluster_offset);
> +        if (ret < 0) {
> +            goto out;
> +        }
> +
> +        offset_in_cluster = offset_into_cluster(s, src_offset);
> +
> +        switch (ret) {
> +        case QCOW2_CLUSTER_UNALLOCATED:
> +            if (bs->backing) {
> +                child = bs->backing;
> +            } else {
> +                flags |= BDRV_REQ_ZERO_WRITE;
> +            }
> +            break;

Do we need a special case if the backing file is shorter than this
image?

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