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

[Andrew Jeffery]

On Thu, 13 May 2021, at 22:30, Jonathan Cameron wrote:
> 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

Much better to use lore.kernel.org:

https://lore.kernel.org/qemu-devel/20170630030058.28943-1-andrew@aj.id.au/

because...

> 
> 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 */

It becomes easier to find Paolo's suggestion to fix that here:

https://lore.kernel.org/qemu-devel/cd1aba90-176f-9ec6-3e2b-d1135156a96d@redhat.com/

Would love to see this resolved! Unfortunately I haven't had the 
bandwidth to fix it all up for ... a long time now.

Andrew