Re: X server in dom0: Bad VBT signature

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

> The abstraction problem is that /dev/mem is used to access both real
> RAM by physical address, and memory-mapped devices.  On real x86 this
> is not an issue because it's the same address space.

I don't really know x86, so it's entirely possible I've got this all
confused.  But I thought there were two I/O address spaces, one
memory-mapped and another one, with a much smaller address space (one
byte?), which uses different instructions.  Am I misremembering?
Confusing with some other CPU?

> On Xen (and maybe other hardware platforms)this is a problem because
> these are distinct addresses spaces, with eventually overlapping
> ranges.

Well, on the SPARC, there are alternative memory access instructions
which take an address space identifier; they're not used for normal
device access, though, perhaps in part because they are privileged-only
and thus can't be used for user-mode device access.  (They're used for
things like accessing the MMU hardware - segment maps, for example, or
cache flushes.)

But I doubt the SPARC is very relevant to Xen per se...though it's
probably not irrelevant to the design of /dev/mem.

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