Re: [RFC][PATCH] new timeofday core subsystem (v. A3)

[Albert Cahalan <albert@users.sf.net>]

On Thu, 2005-03-17 at 16:55 +0000, Russell King wrote:
> On Tue, Mar 15, 2005 at 10:23:54AM -0500, Albert Cahalan wrote:
> > On Mon, 2005-03-14 at 19:22 -0800, Christoph Lameter wrote:
> > > On Mon, 14 Mar 2005, Albert Cahalan wrote:
> > > 
> > > > When the vsyscall page is created, copy the one needed function
> > > > into it. The kernel is already self-modifying in many places; this
> > > > is nothing new.
> > > 
> > > AFAIK this will only works on ia32 and x86_64 and not definitely not
> > > on ia64. Who knows about the other platforms ....
> > 
> > I'll bet it does work fine on IA-64. If it didn't, you would
> > be unable to load the kernel or load an executable.
> > 
> > I know it works for PowerPC. You'll need an isync instruction
> > of course. You may also want a sync instruction and some code
> > to invalidate the cache.
> > 
> > Setting up the page content should be a 1-time operation done
> > at boot. Check your processor manuals as needed.
> 
> Won't work on ARM.  We have XIP kernels, which prevents the use of
> self-modifying code.

Does the ARM kernel provide a special page of code for
apps to execute? If not, then ARM is irrelevant.

Doesn't ARM always have an MMU? If you have an MMU, then
it is no problem to have one single page of non-XIP code
for this purpose.

Supposing that you do support the vsyscall hack and you don't
have an MMU, you can just place the tiny code fragment on the
stack (or anywhere else) when an exec is performed.

So, as far as I can see, ARM is fully capable of supporting this.