From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Message-ID: <17939.11371.206449.628682@cargo.ozlabs.ibm.com> Date: Wed, 4 Apr 2007 14:41:15 +1000 From: Paul Mackerras To: Roland Dreier Subject: Re: [PATCH] Allow drivers to map individual 4k pages to userspace In-Reply-To: References: <17938.14674.621036.256587@cargo.ozlabs.ibm.com> Cc: linuxppc-dev@ozlabs.org, Joachim Fenkes List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Roland Dreier writes: > I assume this is an eHCA-specific problem. Guilty as charged. :) > Another approach is simply not to enable the other 4K pages that are > exposed when mapping a 64K page into userspace - ie only use 1/16th of > the available contexts. Although perhaps eHCA has such a limited # of > contexts that this is not practical. I believe the hypervisor controls the allocation of contexts to partitions, so this wouldn't be practical. > The problem with this approach is that remap_4k_pfn is > powerpc-specific, right? It is powerpc-specific, but so is the eHCA driver... > For example, I don't believe that an ia64 > kernel running with 16K pages could implement this. Which means that > any driver that calls remap_4k_pfn is now powerpc-specific (or has an > #ifdef to work around this). I am a complete ia64-ignoramus, so I couldn't say whether ia64 could do it or not. > In fact my impression was that the powerpc MMU is not part of the > architecture, in the sense that a new implementation could come along > that supported 64K pages without the ability to do this 4K aliasing > trick. Which would make multiplatform kernels very painful, since > remap_4k_pfn might work for some platforms the kernel could boot on. > Or is this not a problem? The PowerPC architecture distinguishes between server and embedded processors. The MMU is part of the architecture for server processors, and is specified reasonably tightly. For embedded there is also a specification of the MMU but it is very loose. In any case, all PowerPC processors can do 4k pages, so I don't see any problem. Paul.