Re: [Qemu-devel] [RFC PATCH RDMA support v4: 08/10] introduce QEMUFileRDMA

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 03/19/2013 05:18 AM, Paolo Bonzini wrote:
> Il 18/03/2013 21:33, Michael R. Hines ha scritto:
>>> +int qemu_drain(QEMUFile *f)
>>> +{
>>> +    return f->ops->drain ? f->ops->drain(f->opaque) : 0;
>>> +}
>>> Hmm, this is very similar to qemu_fflush, but not quite. :/
>>>
>>> Why exactly is this needed?
>> Good idea - I'll replace drain with flush once I added
>> the  "qemu_file_ops_are(const QEMUFile *, const QEMUFileOps *) "
>> that you recommended......
> If I understand correctly, the problem is that save_rdma_page is
> asynchronous and you have to wait for pending operations to do the
> put_buffer protocol correctly.
>
> Would it work to just do the "drain" in the put_buffer operation, if and
> only if it was preceded by a save_rdma_page operation?

Yes, the drain needs to happen in a few places already:

1. During save_rdma_page (if the current "chunk" is full of pages)
2. During the end of each iteration (now using qemu_fflush in my current 
patch)
3. And also during qemu_savem_state_complete(), also using qemu_fflush.
>>>>    /** Flushes QEMUFile buffer
>>>>     *
>>>>     */
>>>> @@ -723,6 +867,8 @@ int qemu_get_byte(QEMUFile *f)
>>>>    int64_t qemu_ftell(QEMUFile *f)
>>>>    {
>>>>        qemu_fflush(f);
>>>> +    if(migrate_use_rdma(f))
>>>> +    return delta_norm_mig_bytes_transferred();
>>> Not needed, and another undesirable dependency (savevm.c ->
>>> arch_init.c).  Just update f->pos in save_rdma_page.
>> f->pos isn't good enough because save_rdma_page does not
>> go through QEMUFile directly - only non-live state goes
>> through QEMUFile ....... pc.ram uses direct RDMA writes.
>>
>> As a result, the position pointer does not get updated
>> and the accounting is missed........
> Yes, I am suggesting to modify f->pos in save_rdma_page instead.
>
> Paolo
>

Would that not confuse the other QEMUFile users?
If I change that pointer (without actually putting bytes
in into QEMUFile), won't the f->pos pointer be
incorrectly updated?