Re: [Qemu-devel] [PATCH 6/10] Introduce v3 of savevm protocol

[Avi Kivity <avi@qumranet.com>]

iAnthony Liguori wrote:
> The current savevm/loadvm protocol has some draw backs.  It does not support
> the ability to do progressive saving which means it cannot be used for live
> checkpointing or migration.  The sections sizes are 32-bit integers which
> means that it will not function when using more than 4GB of memory for a guest.
> It attempts to seek within the output file which means it cannot be streamed.
> The current protocol also is pretty lax about how it supports forward
> compatibility.  If a saved section version is greater than what the restore
> code support, the restore code generally treats the saved data as being in
> whatever version it supports.  This means that restoring a saved VM on an older
> version of QEMU will likely result in silent guest failure.
>
> This patch introduces a new version of the savevm protocol.  It has the
> following features:
>
>  * Support for progressive save of sections (for live checkpoint/migration)
>  * An asynchronous API for doing save
>  * Support for interleaving multiple progressive save sections
>    (for future support of memory hot-add/storage migration)
>  * Fully streaming format
>  * Strong section version checking
>
> Right now, the code is missing to support restoring v2 images.
>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>
>   

> +int qemu_savevm_state_iterate(QEMUFile *f)
> +{
> +    SaveStateEntry *se;
> +    int ret = 0;
> +
> +    for (se = first_se; se != NULL; se = se->next) {
> +        if (se->save_live_state == NULL)
> +            continue;
> +
> +        /* Section type */
> +        qemu_put_byte(f, QEMU_VM_SECTION_PART);
> +        qemu_put_be32(f, se->section_id);
> +
> +        ret |= se->save_live_state(f, QEMU_VM_SECTION_PART, se->opaque);
>   

What if the callback returns an error?

> +    }
> +
> +    if (ret)
> +        return 1;
> +
> +    return 0;
> +}
> +
>   

An alternative solution that doesn't involve iterating over the saveset 
over and over again involves providing a queue of uncompleted state 
saves.  If a save handler has more work to be done, it queues a 
continuation and returns.  The queue is primed by _begin().  
qemu_savevm_state_iterate() would simply attempt to drain the queue.  I 
don't think it's a significant improvement though.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.