Re: [Qemu-devel] RFC: Code fetch optimisation

[Paul Brook <paul@codesourcery.com>]

> Well, we got the same behavior on PowerPC. What I was thinking of is
> that if we fix the VLE problems, the fix, if done in a proper way, could
> also allow benefit to RISC targets. What I don't know is; would we
> really have a benefit not stopping translation on page boundaries ?

> > For VLE targets we already get this wrong (the prefetch abort occurs some
> > time before the faulting instruction executes). I don't know if this
> > behavior is permitted by the ISA, but it's definitely possible to
> > construct cases where it has visible effect.
>
> I think that it would be possible to do things properly.
> [...] Or maybe the solution would
> just be to stop the translation knowing that the exception will be
> raised when trying to translate the first instruction in the next page.

I'd go for this one. It's approximately the same method currently used for 
RISC targets.

In general think this will require target specific support. For RISC targets 
this is trivial. For x86/m68k figuring out the length of an insn is trickier.

Detecting crossing a page boundary on subsequent insns in the load/mmu 
routines is problematic because it happens relatively late. In particular it 
may theoretically happen after we've output ops that change CPU state.

I suspect the best solution is to backtrack (remove the generated ops) after 
decoding the insn if we discover we've passed a page boundary. The ld*_code 
routines can simply return garbage (e.g. zero) if the read is not on the 
first page.

Trying to generate prefetch aborts at runtime sounds too hairy for my liking.

Paul