On 6/22/07, Arjan van de Ven <[email protected]> wrote:
On Fri, 2007-06-22 at 01:56 -0400, Albert Cahalan wrote:
> On 6/21/07, Arjan van de Ven <[email protected]> wrote:
> > 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...
>
> If the instructions to force data write-back and/or to
> invalidate the instruction cache are priveleged, yes.
> AFAIK, only ARM is that lame.
and your program executes this on all the cpus in the system?
I'll remember that if I ever run a JIT on the SMP ARM box.
(there's like one, at the manufacturer, right?)
I don't recall seeing such code in the libgcc tranpoline
setup for PowerPC. Either it's not required, or this is
a rather popular bug.
Perhaps ARM needs syscalls for this, or emulation for
the privileged instructions. This may already exist; it
sure is required. So this would be another need for
properly supporting JIT emulators.
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