Re: [Qemu-devel] [RFC][PATCH] x86: CS limit checks

[Fabrice Bellard <fabrice@bellard.org>]

Paul Brook wrote:
>> To me it looks like as if the generator can so far raise a PF
>> prematurely when it steps on an invalid code address while building a
>> new TB. This probably has to fix the same way as the limit check is
>> realized: by injecting an exception (PF or GP) into the generated code
>> at the correct PC. Hmm, the PF-during-translation issue is probably not
>> just limited to x86...
> 
> Alpha, PPC, SPARC, SH and ARM avoid the problem by having fixed length word 
> aligned instructions. Thumb-1 has special handling for the cross-boundary 
> case (Instructions aren't really variable length, we just treat them that way 
> as an optimisation).
> 
> Thumb-2, m68k, cris and x86 all look like they may incorrectly fetch code from 
> the next page.

For x86 it is an expected behavior, not a bug. However, I agree that it 
would be safer to explicitely generate the exception. My plan has always 
been to suppress the ldx_code functions and to explicitly handle the PF 
and the cs_limit cases. Jocelyn Mayer submitted some time ago a patch to 
go in that direction.

Fabrice.