Re: [PATCH v2] physmem: allow cpu_memory_rw_debug to write to MMIO devices

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Wed, 8 Jan 2025 at 20:10, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 08.01.25 19:35, Stefan Zabka wrote:
>> > On 21/12/2024 15:55, David Hildenbrand wrote:
>> >   > Let's wait for opinions from others first.
>> >
>> > <https://www.qemu.org/docs/master/devel/submitting-a-patch.html#if-your-patch-seems-to-have-been-ignored>
>> > states that two weeks is a reasonable amount of time for follow-up.
>> >
>> > Should I also ping the original patch? I thought pinging the thread
>> > would be more appropriate, as it contains relevant information.
>> >
>>
>> I just pushed a compiling version of the attrs.debug approach to:
>>
>>         https://github.com/davidhildenbrand/qemu/tree/debug_access
>
> I think this approach (having a 'debug' attribute in the MemTxAttrs
> seems reasonable. I do note that if we allow this kind of access
> to write to MMIO devices then we are also permitting ELF (and other)
> image loads to write to MMIO devices where currently we ignore those.
> That means there's probably a class of guest images (of dubious
> correctness) which will start writing junk (likely zeroes) into
> device model registers; we previously would silently ignore any
> such bogus ELF sections.
>
> Q: should we suggest behaviour for device models if they see a
> 'debug = 1' transaction, e.g. "don't update your internal state
> for a debug read if you have clear-on-read or similar kinds of
> register fields" ?

What do we do for device models that want to know which CPU things are
coming from, as per:

  https://gitlab.com/qemu-project/qemu/-/issues/124

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro