Re: [PATCH 0/8] Simplify memory_region_add_subregion_overlap(..., priority=0)

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 16 Dec 2019 at 11:40, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sun, Dec 15, 2019 at 03:27:12PM +0000, Peter Maydell wrote:
> > On Sun, 15 Dec 2019 at 09:51, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Sat, Dec 14, 2019 at 04:28:08PM +0000, Peter Maydell wrote:
> > > > (It doesn't actually assert that it doesn't
> > > > overlap because we have some legacy uses, notably
> > > > in the x86 PC machines, which do overlap without using
> > > > the right function, which we've never tried to tidy up.)
> > >
> > > It's not exactly legacy uses.
> > >
> > > To be more exact, the way the non overlap versions
> > > are *used* is to mean "I don't care what happens when they overlap"
> > > as opposed to "will never overlap".
> >
> > Almost all of the use of the non-overlap versions is
> > for "these are never going to overlap" -- devices or ram at
> > fixed addresses in the address space that can't
> > ever be mapped over by anything else. If you want
> > "can overlap but I don't care which one wins" then
> > that would be more clearly expressed by using the _overlap()
> > version but just giving everything that can overlap there
> > the same priority.
>
> Problem is device doesn't always know whether something can overlap it.
> Imagine device A at a fixed address.
> Guest can program device B to overlap the fixed address.
> How is device A supposed to know this can happen?

That's why (the original intention was) only one of the
regions needs to be marked 'overlap OK', not both.

thanks
-- PMM