Subject: RE: PXA250 Xscale and page tables
To: Jason R Thorpe <email@example.com>
From: Colin Cook <firstname.lastname@example.org>
Date: 02/02/2003 00:48:19
Ok, Sorry I'm slow, are you saying that the I-Cache deals with virtual
addresses only? So if 2 VA's mapped to the same PA, you could write to the
first and it would sit in the cache for that address but VA 2 would not see
it? Doesn't the caches implement some type of snoop-invalidate protocol?
(Again sorry for foolish questions, I don't know ARM all that well).
1) We are dealing with Read Only code pages, So what if it's only in 2
2) How did it get in 2 places in the first place. This is nothing more than
a simple application. Does NetBSD make a habit out of duplicating CODE
3) I could totally see your point if we were dealing with a shared data area
between kernel and user space. But we are dealing with the executable code
of an application.
I am not looking to get into a fight with NetBSD, I just want my program to
run fast. Just tell me the right way to get the program to run out of
From: Jason R Thorpe [mailto:email@example.com]
Sent: Saturday, February 01, 2003 11:51 PM
To: Colin Cook
Subject: Re: PXA250 Xscale and page tables
On Sat, Feb 01, 2003 at 10:13:55AM -0700, Colin Cook wrote:
> Sorry, There *is* something grossly wrong with ALL arm32 based NetBSD
> ptes[arm_btop(npv->pv_va)] &= ~L2_S_CACHE_MASK in pmap.c
> It is removing the C bit from application CODE space. We have removed
> line and our app now runs 10X faster.
The ARM uses a virtually-indexed,virtually-tagged cached. Making the page
un-cached is required if you have multiple mappings of the same physical
page within an address space (either 2 mappings in the same proc or a proc
mapping and a kernel mapping for the same page), thus causing an
cache alias, leading to Bad Things.
-- Jason R. Thorpe <firstname.lastname@example.org>