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

[Jonathan Cameron]

On Thu, 13 May 2021 14:36:27 +0200
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> 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

Thanks - that's very similar to what I was carrying, but I think it
only covers the read case.  That's backed up by the comment:
/* XXX: Can't do this hack for writes */

> 
> > 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).

I'm only aware of this one CXL emulated device (+ the proposed code in
the ADC in the above patch set).  For the CXL device, working around
this limitation is straight forward if that's the right option
+ updating the docs to slightly reduced chances of this being hit in
the future.

Thanks,

Jonathan

> > 
> > thanks
> > -- PMM
> >   
>