On Fri Jun 23, 2023 at 7:10 PM AEST, Peter Maydell wrote: > On Fri, 23 Jun 2023 at 09:21, Nicholas Piggin <npig...@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