Re: [Qemu-devel] [PATCH 0/2] Drop support for qdev taddr properties

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

On 2 April 2013 17:42, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 02/04/2013 18:10, Peter Maydell ha scritto:
>> Aside: I may try to get rid of the DMAADDR property too at some
>> point, because what the sysbus-ohci device should actually be doing
>> is taking a MemoryRegion* [or maybe a DMAContext*]
>
> Avi had patches to unify DMAContext and MemoryRegion.  I should revive
> them perhaps.

Yes please.

>> representing what
>> it should be DMAing into, rather than the current "DMA into the
>> system address space at addr + some constant offset" hack. One
>> thing at a time, though.
>
> That's a general problem with sysbus.  I guess if you need another
> address space you should define your own bus, like PCI does.

It's not a problem with sysbus, it's a problem with people
being lazy about implementing things that do DMA. For instance
PCI doesn't take a MemoryRegion* for DMA, it just assumes it
can DMA into the system address space. (it does let you pass
a DMAContext, but maybe that goes away with the patches you
mention above; it's only used for spapr.)

NB that "the PCI address space", ie the view of the world that
the host controller sees when it makes a PCI access, is not
the same as the address space that another bus mastering PCI
device sees when it does a DMA access. Chances are good that
the former shouldn't have any of the system's RAM in it and
the latter should. (This is easy to handle though, you just
create container MemoryRegions for each and pass them to the
PCI code and make sure that you're using the right one at
any particular point.)

-- PMM