[Qemu-devel] Guest memory mapping in Qemu

[Michael T]

Hello,

This is an idle question in the sense that, much as I would like to, I know for 
a
fact that I won't have the time to look at implementing this.  I'm not expecting
other people to seriously look at doing it either, but I would be interested on 
your
thoughts.

If the technical documentation at
http://www.usenix.org/publications/library/proceedings/usenix05/tech/freenix/full_papers/bellard/bellard_html/index.html
is still valid (I think it is), Qemu has two modes of handling access to guest 
memory -
system emulation, in which an entire guest address space is mapped on the host, 
and
emulated MMU.  I was wondering whether something in-between would also be 
feasible.
That is, chunks of guest address space (say 4MB chunks for the sake of the 
argument)
are mmapped into the address space of the Qemu process on the host, and when an
access to guest memory is made, there is an initial check to see whether it is 
in
the same chunk as the last one, in which case all the MMU emulation bits could 
be
saved.  I could imagine Qemu keeping a current/most recent chunk for each 
register
which can be used for relative addressing, plus one for non-register-relative
accesses.  It seems to me that this could potentially speed up memory access 
quite a
bit, and as a bonus even make it easy to support x86 segmentation (as part of 
the
bounds check for whether a memory access is in a chunk).

I realise of course that I have glibly glossed over all the nasty bits - off 
the top
of my head keeping track of all the mapped chunks in the host address space, 
lookups
to see if an access outside of the current chunk is inside another mapped one,
invalidating chunks when guest page tables they are based on change.  I am sure 
that
there are many more issues...

Looking forward to reading any responses.

Regards,

Michael
                                          
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