Date: Tue, 15 Apr 2008 11:15:22 -0700 From: Marcel Moolenaar <xcllnt@mac.com> To: Rafal Jaworowski <raj@semihalf.com> Cc: grehan@freebsd.org, freebsd-ppc@freebsd.org Subject: Re: kernel stacks [eas: Re: G5 Bridge-mode MMU] Message-ID: <4454B9C3-77FF-489B-AF14-679D66FD2DED@mac.com> In-Reply-To: <4804ED17.3070907@semihalf.com> References: <4804AE13.2060600@uchicago.edu> <4804C9E9.6010303@freebsd.org> <5CC81F06-7B59-4163-9AB8-2ACE4235A5AA@mac.com> <4804DD02.10304@freebsd.org> <058EEFE3-09D7-447A-93AB-3E90EC59ECDC@mac.com> <4804ED17.3070907@semihalf.com>
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On Apr 15, 2008, at 10:59 AM, Rafal Jaworowski wrote: > Marcel Moolenaar wrote: >>>> Related: how hard would it be to map the kernel above 2G and >>>> eliminate the SR swap in the exception handlers (but do it on >>>> context switches)? >>> >>> You would have to eliminate the 1:1 mapping. I know this was done in >>> the bookE port, and it is painful to have two kernel ABIs. >>> >>> How about the corollary: how hard is it to get the bookE port to >>> use a >>> separate address space for the kernel ? >> >> Probably not too hard. >> >> We (i.e. Juniper) discussed this with Semihalf and for Juniper the >> single address space implementation was better, even though Semihalf >> started with the split address space. > > Yeah, in the original design we considered separate address spaces, > but after > discussions we went for the single AS, which apparently is simpler > and cheaper > (no need for any additional temporary mappings in copyin/out, > exceptions and > similar), but imposes limitations on the I/O ranges and KVA. So > maybe with the > upcoming more powerful BookE systems, with lots of RAM etc. it would > be time > for re-considering this, but I don't have any code from back then as > the idea > died pretty quickly, before main development happened. It would be great if we could support both -- not at runtime, but have it compile-time selectable. PowerPC does cover the embedded, desktop and server space so the needs are much more varied than for other platforms. I don't see any immediate consequence for our user-space ABI, so it may not be visible to user space (other than having the stack start at 4GB or so)... -- Marcel Moolenaar xcllnt@mac.com
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