Re: RFC: Memory region accesses where .valid.min_access_size < .impl.min_access_size

[Philippe Mathieu-Daudé]

On 5/13/21 2:23 PM, Peter Maydell wrote:
> On Thu, 13 May 2021 at 12:49, Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
>> My initial suggestion was to fix this by adding the relatively
>> simple code needed in the driver to implement byte read / write,
>> but Ben pointed at the QEMU docs - docs/devel/memory.rst which
>> says
>> "
>> .impl.min_access_size, .impl.max_access_size define the access sizes
>>    (in bytes) supported by the *implementation*; other access sizes will be
>>    emulated using the ones available. For example a 4-byte write will be
>>    emulated using four 1-byte writes, if .impl.max_access_size = 1.
>> "
>>
>> This isn't true when we have the situation where
>> .valid.min_access_size < .imp.min_access_size
>>
>> So change the docs or try to make this work?

See also this patch from Francisco:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg636935.html

And full unaligned access support from Andrew:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg461247.html

> I don't (yet) have a view on what the in-principle right thing
> should be, but in practice: how many devices do we have which
> set .valid.min_access_size < .imp.min_access_size ? If we want
> to change the semantics we'd need to look at those to see if they
> need to be adjusted (or if they're just currently buggy and would
> be fixed by the change).
> 
> thanks
> -- PMM
>