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

From: Peter Maydell
Subject: Re: [RFC 0/3] try to solve the DMA to MMIO issue
Date: Thu, 3 Sep 2020 12:19:37 +0100

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.

> 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.

-- PMM

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