linux-kernel - Re: JIT emulator needs

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Message-ID: <787b0d920706220732hef20f67h6b5d1d57db71ead2@mail.gmail.com>
Date:	Fri, 22 Jun 2007 10:32:11 -0400
From:	"Albert Cahalan" <acahalan@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: JIT emulator needs

On 6/22/07, Arjan van de Ven <arjan@...radead.org> wrote:
> On Fri, 2007-06-22 at 01:56 -0400, Albert Cahalan wrote:
> > On 6/21/07, Arjan van de Ven <arjan@...radead.org> 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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