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

[Jason Wang <jasowang@redhat.com>]

On 2020/9/3 下午7:19, Peter Maydell wrote:
> 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.


As discussed, maybe we can track the pending operations in device itself 
and check it in all the possible entry of device codes (irq, MMIO or 
what ever else). This may be easier for stable backport.

Thanks


>
> thanks
> -- PMM
>