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From: Robert Watson <rwatson@FreeBSD.org>
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To: Alan Cox <alc@rice.edu>
Subject: Re: Unmapped I/O
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Cc: alc@freebsd.org, Konstantin Belousov <kostikbel@gmail.com>,
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On Wed, 19 Dec 2012, Alan Cox wrote:

>> Are the machines that don't have a direct map performance critical? My 
>> expectation is that they are legacy or embedded.  This seems like a great 
>> project to do when the rest of the pieces are stable and fast. Until then 
>> they could just use something like pbufs?
>
> I think the answer to your first question depends entirely on who you are. 
> :-)  Also, at the low-end of the server space, there are many people trying 
> to promote arm-based systems.  While FreeBSD may never run on your arm-based 
> phone, I think that ceding the arm-based server market to others will be a 
> strategic mistake.
>
> Alan
>
> P.S. I think we're moving the discussion to far away from kib's original, so 
> I suggest changing the subject line on any follow ups.

Despite moving the discussion a little further away: MIPS-based systems, a 
direct mapped map segment (e.g., kseg, xkphys, etc) is part of the underlying 
design and doesn't rely on any TLB entries at all.  We run much of the kernel 
from direct map regions to avoid causing TLB pressure.

Robert