Re: [Qemu-devel] [RFC][PATCH] x86: Optional segment type and limit checks - v2

[Paul Brook <paul@codesourcery.com>]

> Some 32-bit guests effectively disable segment range calculations
> checks by setting the maximum limit and zero offset.  Apparently, this
> is faster on some real CPUs too.
>
> Could type checking be done at translation time, including the segment
> types in the translation cache key?

Maybe. If we have a spare hflags bit you could probably use that to indicate 
whether segment limit checks are needed.

> For guests like older Linux, with zero base and non-maximum limit in
> user mode, could limit checking be done by the MMU TLB instead?

Not really. The only resonable way to do this would be to use a very large 
virtual address space, with the high bits being the segment descriptor.  This 
might work for 32-bit targets on 64-bit hosts, but even then it's liable to 
be more pain than it's worth.

Paul