Date: Mon, 31 Mar 2003 03:20:30 -0500 From: Jake Burkholder <jake@locore.ca> To: Peter Jeremy <peterjeremy@optushome.com.au> Cc: cvs-all@FreeBSD.org Subject: Re: HEADS UP Re: cvs commit: src/sys/conf options.i386 src/sys/i386/i386 bios.c locore.s machdep.c mpboot.s pmap.c vm86bios.s vm_machdep.c src/sys/i386/include _types.h bus_at386.h param.h pmap. Message-ID: <20030331082030.GD32298@locore.ca> In-Reply-To: <20030331071239.GA27585@cirb503493.alcatel.com.au> References: <200303300524.h2U5Ora7061852@repoman.freebsd.org> <20030330061301.GC21973@locore.ca> <20030330070723.GE21973@locore.ca> <20030330012410.I3292@odysseus.silby.com> <20030330201113.GA32298@locore.ca> <20030330152920.D6586@odysseus.silby.com> <20030330232030.GB32298@locore.ca> <20030331071239.GA27585@cirb503493.alcatel.com.au>
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Apparently, On Mon, Mar 31, 2003 at 05:12:39PM +1000, Peter Jeremy said words to the effect of; > On Sun, Mar 30, 2003 at 06:20:30PM -0500, Jake Burkholder wrote: > >Apparently, On Sun, Mar 30, 2003 at 03:30:52PM -0600, > > Mike Silbersack said words to the effect of; > >> Is it practically possible with PAE and busdma'd drivers that such a > >> configuration could work? > > > >I'm not sure I understand the question, you mean is it possible to use > >separate address spaces for the kernel and userland, giving a full 4G each? > >Yes it is possible, but it is not practical. > > Why do you say "not practical"? Unix spent most of its formative > years with kernel and userland in separate address spaces. I don't > think the code exists in 4BSD but it's definitely still functional > in 2BSD (which is under a BSD license now-a-days). x86 just doesn't lend itself well to doing this. The sparc64 port uses separate address spaces because it is easy there. Its not a matter of the os supporting it, its what you can do realistically with the hardware. > > > It would be prohibitively expensive and ugly. > > I'll accept "ugly" and "expensive". The "prohibitively" is more of a > value judgement. Currently FreeBSD needs to trade KVA against UVA for > large RAM configurations. If you have an application that needs lots > of KVA and lots of UVA then FreeBSD on x86 isn't currently an option. > Wearing the overheads on system calls, copyin and copyout may be > cheaper than the alternatives. Don't let me stop you from implementing it. Jake
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