Define an API so that devices can register MemoryRegionOps whose read
and write callback functions are passed an arbitrary pointer to some
transaction attributes and can return a success-or-failure status code.
This will allow us to model devices which:
* behave differently for ARM Secure/NonSecure memory accesses
* behave differently for privileged/unprivileged accesses
* may return a transaction failure (causing a guest exception)
for erroneous accesses
This patch defines the new API and plumbs the attributes parameter through
to the memory.c public level functions io_mem_read() and io_mem_write(),
where it is currently dummied out.
The success/failure response indication is also propagated out to
io_mem_read() and io_mem_write(), which retain the old-style
boolean true-for-error return.
Backports commit cc05c43ad942165ecc6ffd39e41991bee43af044 from qemu
