From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from cthulhu.engr.sgi.com (cthulhu.engr.sgi.com [192.26.80.2]) by neteng.engr.sgi.com (980205.SGI.8.8.8/970903.SGI.AUTOCF) via ESMTP id NAA2659928 for ; Thu, 2 Apr 1998 13:42:09 -0800 (PST) Return-Path: Received: (from majordomo-owner@localhost) by cthulhu.engr.sgi.com (980205.SGI.8.8.8/970903.SGI.AUTOCF) id NAA7069231 for linux-list; Thu, 2 Apr 1998 13:41:08 -0800 (PST) Received: from fir.engr.sgi.com (fir.engr.sgi.com [150.166.49.183]) by cthulhu.engr.sgi.com (980205.SGI.8.8.8/970903.SGI.AUTOCF) via SMTP id NAA7002252; Thu, 2 Apr 1998 13:41:03 -0800 (PST) Received: (from wje@localhost) by fir.engr.sgi.com (950413.SGI.8.6.12/950213.SGI.AUTOCF) id NAA01565; Thu, 2 Apr 1998 13:41:02 -0800 Date: Thu, 2 Apr 1998 13:41:02 -0800 Message-Id: <199804022141.NAA01565@fir.engr.sgi.com> From: "William J. Earl" To: ralf@uni-koblenz.de Cc: linux@cthulhu.engr.sgi.com Subject: Re: VCE exceptions In-Reply-To: <19980402225314.63238@uni-koblenz.de> References: <19980402225314.63238@uni-koblenz.de> Sender: owner-linux@cthulhu.engr.sgi.com Precedence: bulk ralf@uni-koblenz.de writes: > I think I know why we're catching VCE exceptions even though we try to > avoid them at any price - the reason spells ``empty_zero_page''. This > page is filled with zeros and is being mapped to arbitrary addresses > at the same time. Arbitrary addresses means also bits 14:12 of the > virtual address may be different, welcome VCED. This also means that > at least sane code should never cause VCEI exceptions. The text of > the panic message ``should not happend'' is therefore wrong as well ... > > Whatever, the fact that the hardware causes VCE exceptions which don't > help us at all forces us to handle them somehow. How handy, they'll > fit quite well in the revamped interface for board caches :-) > > Another way to finally eleminate the virtual coherency problem from > KSEG0's landscape would be to actually use 8 pages as an array of > empty_zero_pages[], so we would be able to map one wherever we want > such that we never run into virtual coherency trouble. For an always-zero page, this is the best solution. At a small cost in memory, you get far less overhead.