Re: [Qemu-devel] [PATCH v2 14/16] block/mirror: Add active mirroring

[Max Reitz]

On 2018-02-27 10:34, Fam Zheng wrote:
> 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?

Now that you mention it...

Max

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