Re: JIT emulator needs

[Arjan van de Ven <arjan@infradead.org>]

On Fri, 2007-06-08 at 02:35 -0400, Albert Cahalan wrote:
> Right now, Linux isn't all that friendly to JIT emulators.
> Here are the problems and suggestions to improve the situation.
> 
> There is an SE Linux execmem restriction that enforces W^X.
> Assuming you don't wish to just disable SE Linux, there are
> two ugly ways around the problem. You can mmap a file twice,
> or you can abuse SysV shared memory. The mmap method requires
> that you know of a filesystem mounted rw,exec where you can
> write a very large temporary file. This arbitrary filesystem,
> rather than swap space, will be the backing store. The SysV
> shared memory method requires an undocumented flag and is
> subject to some annoying size limits. Both methods create
> objects that will fail to be deleted if the program dies
> before marking the objects for deletion.


and these methods also destroy yourself on any machine with a looser
cache coherency between I and D-cache....

for all but x86 you pretty much have to do the mprotect() between the
two states to deal with the cache flushing properly...