From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 4C0DDB70AA for ; Tue, 5 Oct 2010 19:09:27 +1100 (EST) Subject: Re: use of BAT before taking over the MMU From: Benjamin Herrenschmidt To: Albert Cahalan In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Tue, 05 Oct 2010 19:09:19 +1100 Message-ID: <1286266159.2463.335.camel@pasglop> Mime-Version: 1.0 Cc: linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Sat, 2010-10-02 at 14:32 -0400, Albert Cahalan wrote: > On the prom boot path, with the firmware supposed to > be managing the MMU, there is a case where: > > 1. Linux changes some BAT registers. > 2. Bits 0x00000070 are/become set in the MSR. > 3. Linux takes an MMU fault. Meeep ! Linux should never take an MMU fault at that point :-) If it does, then there's a bug somewhere that needs squashing. > 4. The firmware handles it. > > AFAIK, you can't expect the firmware to leave the BAT alone. > If the firmware provides mapping services by using the BAT > as a software-filled TLB, Linux's BAT changes may be lost. > > You also can't expect that your BAT changes will not conflict > with mappings that the firmware uses for itself. The firmware > might write to your new BAT mapping, relying on those virtual > addresses to be something else entirely. Right, which is why the moment Linux takes over the BATs, it shouldn't call into FW anymore nor take faults. Cheers, Ben.