Re: [RFC 0/3] try to solve the DMA to MMIO issue

[Li Qiang]

Peter Maydell <peter.maydell@linaro.org> 于2020年9月3日周四 下午7:19写道：
>
> On Thu, 3 Sep 2020 at 12:11, Li Qiang <liq3ea@gmail.com> wrote:
> >
> > Peter Maydell <peter.maydell@linaro.org> 于2020年9月3日周四 下午6:53写道：
> > >
> > > On Thu, 3 Sep 2020 at 04:55, Jason Wang <jasowang@redhat.com> wrote:
> > > > I think we still need to seek a way to address this issue completely.
> > > >
> > > > How about adding a flag in MemoryRegionOps and detect the reentrancy
> > > > through that flag?
> > >
> > > This won't catch everything. Consider this situation:
> > >   Device A makes DMA access to device B
> > >   Device B's write-handling causes it to raise an
> > >    outbound qemu_irq signal
> > >   The qemu_irq signal is connected to device A
> >
> > Here mean device A is an interrupt controller?
>
> No. Any device can have an inbound or outbound qemu_irq line.
> We use them not just for actual IRQ lines but for any
> situation where we need to pass an on-or-off signal from
> one device to another.

Could you please provide some example, I haven't noticed this before. Thanks.

>
> > This is special case I think.
>
> It's an example of why looking purely at MMIO is not
> sufficient. We should prefer to see if we can come up with
> a design principle that works for all between-device
> coordination before we implement something that is specific
> to MMIO.

So first we may be define a clean boundary/interface between device
coordination?

Thanks,
Li Qiang

>
> thanks
> -- PMM