[SeaBIOS] varlow/extrastack vs code

[Gerd Hoffmann]

  Hi,

> Just for historical perspective - the reason I think qemu didn't
> implement the pam "read from rom and write to memory" mode is that I
> don't think there's a good way to emulate that with page tables (and
> the range needs to be executable so just making it all device memory
> isn't practical).

A while back kvm got support for memory slots which allow read access
and vmexit on write access.  Flash emulation needs this, and I think it
is possible to also implement the "read from rom and write to memory"
mode using that.

cheers,
  Gerd