Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 5/8] hw/timer: Add basic M41T80 emulation

[David Gibson]

On Thu, Jun 14, 2018 at 09:54:41AM +0200, BALATON Zoltan wrote:
> On Thu, 14 Jun 2018, David Gibson wrote:
> > On Wed, Jun 13, 2018 at 04:13:57PM +0200, BALATON Zoltan wrote:
> > > I don't see the problem. The addr register selects the register to read or
> > > write. It is set by the first write when the device is accessed the first
> > > time (this is denoted by addr == -1 (or really any negative value). The
> > > device has 20 registers and trying to read any register outside addr 
> > > between
> > > 0-19 will result in returning 0 and logging a warning about invalid 
> > > register
> > > in m41t80_recv. What could fail here when guest sends garbage? It will set
> > > addr to invalid value and try to read non-exitent register and get an 
> > > error
> > > just like for any other nonexistent value of addr (or start to read from
> > > register 0 if it manages to set a negative value). All writes of registers
> > > are ignored currently (except setting addr by the first write). What 
> > > should
> > > be enforced here?
> > 
> > Ah, I see your point.  I mean strictly we should match the hardware
> > behaviour if you write garbage addresses here, but really I don't
> > think it matters much.
> 
> Problem is like usual I have no idea what the real hardware does. I've only
> seen the datasheet, never seen real device and have no way to test. All the
> clients I've tested seem to be OK with the current emulation so unless
> someone has more info on how this should work I think we can live with this
> version and then fix it later if found to be needed.

Ok, fair enough.

-- 
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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