Subject: Re: Kernel copyin/out optimizations for ARM...
To: David Laight <firstname.lastname@example.org>
From: Richard Earnshaw <email@example.com>
Date: 03/14/2002 01:31:26
> > The PTE check is actually checking for the 'copy on write' case,
> > not the 'page not present' case. This may have something to do
> > with the problems Jason? was having with COW on XSCALE.
> > My guess is that the cpu wasn't faulting the write to cache! Just the
> > writeback of the cacheline - which would be asynchonous!
> Further thoughts:
> 1) if this check is needed here, presuambly there are equivalent
> problems for COW for writes from user space?
I think the code is written the way it is because the ARM pte mappings
require that user-read => kernel-write. However, ldrt would solve that
> 2) is address space reserved for PTE entries for the entire user
> address space?
yes and no. L1 space is allocated (4 pages), but L2 space is only
allocated if there are mappings in that region.