Re: [PATCH v2 1/2] system/memory: Use qemu_ram_{copy, move}() in ram device region accessors

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 15, 2026 at 11:57:09AM +0100, Peter Maydell wrote:
> On Mon, 15 Jun 2026 at 11:03, Gavin Shan <gshan@redhat.com> wrote:
> >
> > All ram device regions were turned to be indirectly accessible by commit
> > 4a2e242bbb ("memory: Don't use memcpy for ram_device regions"). This leads
> > to guest hang on attempt to build 'cuda-samples' as reported by Julia. The
> > guest is started by the following command lines, with GH100 GPU card passed
> > from the host.
> 
> > diff --git a/include/system/memory.h b/include/system/memory.h
> > index 1417132f6d..5878727d09 100644
> > --- a/include/system/memory.h
> > +++ b/include/system/memory.h
> > @@ -2897,6 +2897,8 @@ void address_space_register_map_client(AddressSpace *as, QEMUBH *bh);
> >  void address_space_unregister_map_client(AddressSpace *as, QEMUBH *bh);
> >
> >  /* Internal functions, part of the implementation of address_space_read.  */
> > +void qemu_ram_copy(void *dest, const void *src, size_t n);
> > +void qemu_ram_move(void *dest, const void *src, size_t n);
> 
> New function prototypes in include headers need documentation comments.
> 
> In particular for these, it's really important that we clearly say
> what semantics we are attempting to provide with them, so that
> (a) when we're reviewing or later updating the implementation we
> know what we are trying to provide
> (b) when we're looking at the callsites we know what the function
> is guaranteeing to us and what it is not, and thus whether it's
> OK to use it or we need something els.
> 
> > +#if defined(__i386__) || defined(__x86_64__)
> > +#define HOST_UNALIGNED_MMIO_OK 1
> > +#else
> > +#define HOST_UNALIGNED_MMIO_OK 0
> > +#endif
> 
> We need to do something better than this. We can't
> just say "oh, we trust that on x86 this works": it is
> neither actually true that the compiler guarantees it even
> on x86, nor is it the case that only x86 can do unaligned
> accesses to normal RAM.
> 
> thanks
> -- PMM

Why normal RAM? It explicitly says MMIO.
x86 seems to be the only arch that can do unaligned MMIO?