Re: X server in dom0: Bad VBT signature

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> it's the memory-mapped address space which is causing problem for
> Xen, because the physical addresses are not real machine addresses,
> they are translated by the hypervisor, and may have the same address
> as real machine address but point to something different.

Sounds as though Xen is the problem, in that it is putting two
different things (RAM and memory-mapped hardware) at the same
(emulated-)physical address.  Or am I still misunderstanding?

>> Well, on the SPARC, there are alternative memory access instructions
>> which take an address space identifier;
> Can address space identifier be used by non-privilegied instructions?

As far as I can tell it cannot; the instructions in question (sta, lda,
stba, lduba, etc) are, I think, defined to fault if attempted, at all,
from nonprivileged mode.

> If so, it could be an example for an extended /dev/mem

Maybe.  I'm not sure what your extended /dev/mem's interface would look
like.

As I understand it, it's not possible to set things up so that there is
a virtual address that, when used with the ordinary load/store
instructions, turns into alternative address space accesses; this means
that there is nothing mmap() could possibly return that would look like
memory but access an alternative address space.  The only way I can see
to do this would be to return a no-access page to userland and trap to
emulator code in the kernel for every access.

It would, concetpually, be possible for the pmap to maintain multiple
address spaces for a process, and have some kind of asimmap() that
returns a `pointer' useful only with the correct ASI value and
userland-usable lda/sta/etc instructions.  But those instructions are
only hypothetical, so I see no benefit to following this line of
thought, unless perhaps some other architecture has similar facilities
that are usable by userland.  Furthermore, as far as I know, the ASI
memory access instructions have nothing like an MMU between the address
and what it accesses, making asimmap() not only impossible but also
unnecessary.

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