Re: arch/xen is a bad idea

[Ian Pratt <Ian.Pratt@cl.cam.ac.uk>]

> > There are so many problems in snooping and decoding instructions it
> > isn't funny. Aside from the mmap pci buffer half way through instruction
> 
> Hmm? From what I remember reading the code of the Xen hypervisor
> they are already emulating a lot of instructions (e.g. take a look
> at xen/arch/x86/x86_32/seg_fixup.c) Emulating some more doesn't 
> seem like a big issue to me.

There's actually no emulation in Xen at all. seg_fixup is the
closest we come, which is a tiny decoder that is able to work out
the effective address of an instruction. It's only through grim
necessity we need this in order to be able to make NPTL thread
local storage accesses work (-ve segment offsets). We'd much
prefer that glibc was slightly tweaked such that we didn't have
to do this (it would be a lot faster too).

Anyhow, I really don't think emulation is the issue. The handful
of cases that you pointed out that could relatively easily be
emulated constitute a tiny fraction of the arch xen patch.  Even
so, you have to be a bit careful from a performance point of
view: loading debug registers can occur quite frequently, and its
much quicker to be able to load them as a batch rather than
taking individual faults. 


Ian