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

[Michael R. Hines]

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?