Re: [Qemu-ppc] [PATCH v6 07/15] spapr_rtas: add ibm, configure-connector RTAS interface

[David Gibson]

On Wed, Mar 04, 2015 at 07:37:08AM -0600, Michael Roth wrote:
> Quoting Michael Roth (2015-03-03 23:50:34)
> > Quoting David Gibson (2015-03-02 23:33:39)
> > > On Mon, Mar 02, 2015 at 10:40:16PM -0600, Michael Roth wrote:
> > > > Quoting David Gibson (2015-03-02 01:02:46)
> > > > > On Thu, Feb 26, 2015 at 09:11:07PM -0600, Michael Roth wrote:
> > > > > > This interface is used to fetch an OF device-tree nodes that 
> > > > > > describes a
> > > > > > newly-attached device to guest. It is called multiple times to walk 
> > > > > > the
> > > > > > device-tree node and fetch individual properties into a 
> > > > > > 'workarea'/buffer
> > > > > > provided by the guest.
> > > > > > 
> > > > > > The device-tree is generated by QEMU and passed to an 
> > > > > > sPAPRDRConnector during
> > > > > > the initial hotplug operation, and the state of these RTAS calls is 
> > > > > > tracked by
> > > > > > the sPAPRDRConnector. When the last of these properties is 
> > > > > > successfully
> > > > > > fetched, we report as special return value to the guest and 
> > > > > > transition
> > > > > > the device to a 'configured' state on the QEMU/DRC side.
> > > > > > 
> > > > > > See docs/specs/ppc-spapr-hotplug.txt for a complete description of
> > > > > > this interface.
> > > > > > 
> > > > > > Signed-off-by: Michael Roth <address@hidden>
> > > > > 
> > > > > 
> > > > > So, actually, here's probably the best place to explain what I had in
> > > > > mind for changing the internal interface for this stuff.  I was
> > > > > thinking something like this pseudocode:
> > > > > 
> > > > > struct DRCCCState {
> > > > >         void *fdt;
> > > > >         int offset;
> > > > >         int depth;
> > > > > };
> > > > > 
> > > > > rtas_configure_connector()
> > > > > {
> > > > >         ...
> > > > >         DRCCCState *ccstate;
> > > > >         ...
> > > > > 
> > > > >         /* check parameters, retrieve drc */
> > > > >         ccstate = drc->ccstate;
> > > > > 
> > > > >         if (!ccstate) {
> > > > >                 /* Haven't started configuring yet */
> > > > >                 ccstate = malloc(...);
> > > > >                 /* Retrieve the dt fragment from the backend */
> > > > >                 ccstate->fdt = drck->get_dt(...);
> > > > >                 ccstate->offset = 0;
> > > > >         }
> > > > > 
> > > > >         while (get next tag from fdt) {
> > > > >                 switch (tag)
> > > > >                 case FDT_PROPERTY:
> > > > >                         /* Translate property into rtas return values 
> > > > > */
> > > > >                         return SPAPR_DR_CC_RESPONSE_NEXT_PROPERTY;
> > > > > 
> > > > >                 /* other cases ... */
> > > > >         }
> > > > >         
> > > > >         /* Fall through only if we've completed streaming out the dt
> > > > >         */
> > > > > 
> > > > >          /* Tell the back end we've finished configuring */
> > > > >         drck->cc_completed(...);
> > > > >         return SPAPR_DR_CC_RESPONSE_SUCCESS;
> > > > > }
> > > > > 
> > > > > On reset, or anything else which interrupts the configuration process,
> > > > > just blow away drc->ccstate.
> > > > 
> > > > Ok, that seems reasonable. I took a stab at it here:
> > > > 
> > > >     
> > > > https://github.com/mdroth/qemu/commit/79ce372743da1b63a6fa33e3de1f1daba8ea1fdc
> > > >     https://github.com/mdroth/qemu/commits/spapr-hotplug-pci
> > > 
> > > It's looking pretty close now, thanks for the rework.
> > > 
> > > > It exposes the ccstate as you suggested, via drck->get_cc_state(), and 
> > > > in
> > > > place of drck->cc_completed() I have drck->set_configured() which serves
> > > > roughly the same purpose I think. I opted not to let RTAS handle
> > > > allocation, since it seemed to imply RTAS owns it and not the DRC.
> > > 
> > > So, that was intentional; basically RTAS *does* own the CCstate.  But
> > > for convenience of index we need connect it to the DRC.  Think of it
> > > like an rtas_priv field in the DRC.
> > > 
> > > In particular I think the CCstate should be opaque to everything
> > > except the RTAS code itself, which means initializing the offset and
> > > depth in RTAS, not in a drck callback.  As far as the drck callback
> > > is concerned, it's supplying a dt fragment, but it doesn't care about
> > > the details of how the upper layer communicates that through to the
> > > guest.
> > 
> > Ah ok, so it was about moving the CCState out of DRC, and not just the
> > awkward interface that wraps FDT traversal. So I went ahead and did it
> > as you suggested, but also making it actually opaque, and relying on
> > a couple callbacks that configure-connector passes to
> > drc->begin_configure_connector to handle init/reset of the CCState
> > fields (such as the fdt, and the start offset (which isn't necessarilly 0)):
> > 
> >   https://github.com/mdroth/qemu/commits/spapr-hotplug-pci
> >   
> > https://github.com/mdroth/qemu/commit/732aa10fa2e41951c396373e7df7d31861322531
> > 
> > I think I have all your other comments addressed, so if that looks ok
> > I'll post v7 soon. Thanks!
> 
> Yikes, just noticed a use-after-free in the new code. Fixed here:
> 
>   
> https://github.com/mdroth/qemu/commit/3fd03f649dc5cd34aa6e2544d38855dd0f8b3708

Ok, I'm now getting myself a bit tangled in the various revisions.
However looking at

https://github.com/mdroth/qemu/commit/732aa10fa2e41951c396373e7df7d31861322531

The ->begin_configure_connector stuff seems unnecessarily
complicated.  Couldn't you just have begin_configure_connector()
return the fdt, then initialize ccs in rtas_ibm_configure_connector()
itself, avoiding the callback-from-a-callback.

I'm also not sure that reset_ccs is worth abstracting.  I think it
would be reasonable just to say that freeing and setting to NULL the
ccs link is sufficient.

That said, the current reset_ccs doesn't appear to be quite right,
since it frees the ccs structure, but not the fdt fragment it points
to.  I'm not sure how awkward it would be to force them into a common
allocation to avoid that.

-- 
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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