Unmapped I/O

[Warner Losh]

On Dec 25, 2012, at 1:48 PM, Konstantin Belousov wrote:

> On Tue, Dec 25, 2012 at 08:42:27PM +0000, Robert Watson wrote:
>> 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.
> Yes, as it was noted already, 32bit mips kseg is not much usable on
> the mips systems with more than 1GB of RAM. But Alan' another patch,
> with, I believe, small modification, could provide the gain there too.

Most mips move to 64-bit when they have more than 512MB or 1GB because the direct map is just too handy...

Warner