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

[Paolo Bonzini <pbonzini@redhat.com>]

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?

> 
>>>   /** 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