Re: [Qemu-devel] [PATCH v7 19/42] MIG_CMD_PACKAGED: Send a packaged chunk of migration stream

[Amit Shah]

On (Tue) 16 Jun 2015 [11:26:32], Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> MIG_CMD_PACKAGED is a migration command that wraps a chunk of migration
> stream inside a package whose length can be determined purely by reading
> its header.  The destination guarantees that the whole MIG_CMD_PACKAGED
> is read off the stream prior to parsing the contents.
> 
> This is used by postcopy to load device state (from the package)
> while leaving the main stream free to receive memory pages.

Not sure why this is necessary.  I suppose I'll have to go read the
documentation in patch 1..

However:

> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -718,6 +718,50 @@ void qemu_savevm_send_open_return_path(QEMUFile *f)
>      qemu_savevm_command_send(f, MIG_CMD_OPEN_RETURN_PATH, 0, NULL);
>  }
>  
> +/* We have a buffer of data to send; we don't want that all to be loaded
> + * by the command itself, so the command contains just the length of the
> + * extra buffer that we then send straight after it.
> + * TODO: Must be a better way to organise that
> + *
> + * Returns:
> + *    0 on success
> + *    -ve on error
> + */
> +int qemu_savevm_send_packaged(QEMUFile *f, const QEMUSizedBuffer *qsb)
> +{
> +    size_t cur_iov;
> +    size_t len = qsb_get_length(qsb);
> +    uint32_t tmp;
> +
> +    if (len > MAX_VM_CMD_PACKAGED_SIZE) {
> +        error_report("%s: Unreasonably large packaged state: %zu",
> +                     __func__, len);
> +        return -1;
> +    }
> +
> +    tmp = cpu_to_be32(len);
> +
> +    trace_qemu_savevm_send_packaged();
> +    qemu_savevm_command_send(f, MIG_CMD_PACKAGED, 4, (uint8_t *)&tmp);
> +
> +    /* all the data follows (concatinating the iov's) */
> +    for (cur_iov = 0; cur_iov < qsb->n_iov; cur_iov++) {
> +        /* The iov entries are partially filled */
> +        size_t towrite = (qsb->iov[cur_iov].iov_len > len) ?
> +                              len :
> +                              qsb->iov[cur_iov].iov_len;

If iov_len was > len, we only wrote part of the current buffer, and we
skip to the next?


                Amit