Re: [Qemu-devel] [Qemu-ppc] [PATCH v9 4/6] hw/ppc/spapr.c: migrate pending_dimm_unplugs of spapr state

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 12, 2017 at 04:54:57PM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 05/12/2017 03:12 AM, David Gibson wrote:
> > On Fri, May 05, 2017 at 05:47:44PM -0300, Daniel Henrique Barboza wrote:
> > > To allow for a DIMM unplug event to resume its work if a migration
> > > occurs in the middle of it, this patch migrates the non-empty
> > > pending_dimm_unplugs QTAILQ that stores the DIMM information
> > > that the spapr_lmb_release() callback uses.
> > > 
> > > It was considered an apprach where the DIMM states would be restored
> > > on the post-_load after a migration. The problem is that there is
> > > no way of knowing, from the sPAPRMachineState, if a given DIMM is going
> > > through an unplug process and the callback needs the updated DIMM State.
> > > 
> > > We could migrate a flag indicating that there is an unplug event going
> > > on for a certain DIMM, fetching this information from the start of the
> > > spapr_del_lmbs call. But this would also require a scan on post_load to
> > > figure out how many nr_lmbs are left. At this point we can just
> > > migrate the nr_lmbs information as well, given that it is being calculated
> > > at spapr_del_lmbs already, and spare a scanning/discovery in the
> > > post-load. All that we need is inside the sPAPRDIMMState structure
> > > that is added to the pending_dimm_unplugs queue at the start of the
> > > spapr_del_lmbs, so it's convenient to just migrated this queue it if it's
> > > not empty.
> > > 
> > > Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
> > NACK.
> > 
> > As I believe I suggested previously, you can reconstruct this state on
> > the receiving side by doing a full scan of the DIMM and LMB DRC states.
> 
> Just had an idea that I think it's in the line of what you're suggesting.
> Given
> that the information we need is only created in the spapr_del_lmbs
> (as per patch 1), we can use the absence of this information in the
> release callback as a sort of a flag, an indication that a migration got
> in the way and we need to reconstruct the nr_lmbs states again, using
> the same scanning function I've used in v8.
> 
> The flow would be like this (considering the changes in the
> previous 3 patches so far):
> 
> ------------
> 
> /* Callback to be called during DRC release. */
> void spapr_lmb_release(DeviceState *dev)
> {
>      HotplugHandler *hotplug_ctrl;
> 
>      uint64_t addr = spapr_dimm_get_address(PC_DIMM(dev));
>      sPAPRMachineState *spapr = SPAPR_MACHINE(qdev_get_machine());
>      sPAPRDIMMState *ds = spapr_pending_dimm_unplugs_find(spapr, addr);
> 
>     // no DIMM state found in spapr - re-create it to find out how may LMBs
> are left
>     if (ds == NULL) {
>         uint32 nr_lmbs  = ***call_scanning_LMB_DRCs_function(dev)***
>         // recreate the sPAPRDIMMState element and add it back to spapr
>     }
> 
>     ( resume callback as usual )
> 
> -----------

Yes, the above seems like a reasonable plan.

> Is this approach be adequate? Another alternative would be to use another
> way of detecting if an LMB unplug is happening and, if positive, do the same
> process in the post_load(). In this case I'll need to take a look in the
> code and
> see how we can detect an ongoing unplug besides what I've said above.

You could, but I think the lazy approach above is preferable.

> 
> Thanks,
> 
> 
> Daniel
> 
> > 
> > > ---
> > >   hw/ppc/spapr.c | 31 +++++++++++++++++++++++++++++++
> > >   1 file changed, 31 insertions(+)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index e190eb9..30f0b7b 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -1437,6 +1437,36 @@ static bool version_before_3(void *opaque, int version_id)
> > >       return version_id < 3;
> > >   }
> > > +static bool spapr_pending_dimm_unplugs_needed(void *opaque)
> > > +{
> > > +    sPAPRMachineState *spapr = (sPAPRMachineState *)opaque;
> > > +    return !QTAILQ_EMPTY(&spapr->pending_dimm_unplugs);
> > > +}
> > > +
> > > +static const VMStateDescription vmstate_spapr_dimmstate = {
> > > +    .name = "spapr_dimm_state",
> > > +    .version_id = 1,
> > > +    .minimum_version_id = 1,
> > > +    .fields = (VMStateField[]) {
> > > +        VMSTATE_UINT64(addr, sPAPRDIMMState),
> > > +        VMSTATE_UINT32(nr_lmbs, sPAPRDIMMState),
> > > +        VMSTATE_END_OF_LIST()
> > > +    },
> > > +};
> > > +
> > > +static const VMStateDescription vmstate_spapr_pending_dimm_unplugs = {
> > > +    .name = "spapr_pending_dimm_unplugs",
> > > +    .version_id = 1,
> > > +    .minimum_version_id = 1,
> > > +    .needed = spapr_pending_dimm_unplugs_needed,
> > > +    .fields = (VMStateField[]) {
> > > +        VMSTATE_QTAILQ_V(pending_dimm_unplugs, sPAPRMachineState, 1,
> > > +                         vmstate_spapr_dimmstate, sPAPRDIMMState,
> > > +                         next),
> > > +        VMSTATE_END_OF_LIST()
> > > +    },
> > > +};
> > > +
> > >   static bool spapr_ov5_cas_needed(void *opaque)
> > >   {
> > >       sPAPRMachineState *spapr = opaque;
> > > @@ -1535,6 +1565,7 @@ static const VMStateDescription vmstate_spapr = {
> > >       .subsections = (const VMStateDescription*[]) {
> > >           &vmstate_spapr_ov5_cas,
> > >           &vmstate_spapr_patb_entry,
> > > +        &vmstate_spapr_pending_dimm_unplugs,
> > >           NULL
> > >       }
> > >   };
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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