Re: [PATCH 0/4] target/ppc: Catch invalid real address accesses

[Nicholas Piggin]

On Fri Jun 23, 2023 at 7:10 PM AEST, Peter Maydell wrote:
> On Fri, 23 Jun 2023 at 09:21, Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > ppc has always silently ignored access to real (physical) addresses
> > with nothing behind it, which can make debugging difficult at times.
> >
> > It looks like the way to handle this is implement the transaction
> > failed call, which most target architectures do. Notably not x86
> > though, I wonder why?
>
> Much of this is historical legacy. QEMU originally had no
> concept of "the system outside the CPU returns some kind
> of bus error and the CPU raises an exception for it".
> This is turn is (I think) because the x86 PC doesn't do
> that: you always get back some kind of response, I think
> -1 on reads and writes ignored. We added the do_transaction_failed
> hook largely because we wanted it to give more accurate
> emulation of this kind of thing on Arm, but as usual with new
> facilities we left the other architectures to do it themselves
> if they wanted -- by default the behaviour remained the same.
> Some architectures have picked it up; some haven't.
>
> The main reason it's a bit of a pain to turn the correct
> handling on is because often boards don't actually implement
> all the devices they're supposed to. For a pile of legacy Arm
> boards, especially where we didn't have good test images,
> we use the machine flag ignore_memory_transaction_failures to
> retain the legacy behaviour. (This isn't great because it's
> pretty much going to mean we have that flag set on those
> boards forever because nobody is going to care enough to
> investigate and test.)
>
> > Other question is, sometimes I guess it's nice to avoid crashing in
> > order to try to quickly get past some unimplemented MMIO. Maybe a
> > command line option or something could turn it off? It should
> > probably be a QEMU-wide option if so, so that shouldn't hold this
> > series up, I can propose a option for that if anybody is worried
> > about it.
>
> I would not recommend going any further than maybe setting the
> ignore_memory_transaction_failures flag for boards you don't
> care about. (But in an ideal world, don't set it and deal with
> any bug reports by implementing stub versions of missing devices.
> Depends how confident you are in your test coverage.)

Thanks for the background, interesting and helpful. So I think
it is the right place for powerpc BookS 64 to hook into. Point
taken about adding a global option for it. Will try to fix the
known problems first, maybe it won't be too hard.

Thanks,
Nick