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Re: does the SAVE_ALL nesting in kernel?

To: "zhuzhenhua" <zzh.hust@gmail.com>, "linux-mips" <linux-mips@linux-mips.org>
Subject: Re: does the SAVE_ALL nesting in kernel?
From: "Kevin D. Kissell" <kevink@mips.com>
Date: Wed, 12 Sep 2007 11:11:42 +0200
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <50c9a2250709111921g1b49cb0du7f97ebb3e1fb7d07@mail.gmail.com>
Sender: linux-mips-bounce@linux-mips.org
SAVE_ALL can certainly nest in MIPS Linux.  This is why there's a check at the beginning of SAVE_SOME to determine whether the exception was take in user-mode, to see if the kernel stackpointer should be loaded from memory or computed relative to the current stack frame.   Another thing to keep in mind in looking at these sorts of optimizations is that each scheduling thread has its own kernel stack area. So if you're very very motivated, you could conceivably come up with a hack whereby the first level of exceptions uses your internal RAM array for a stack, but nested ones use external memory, *but*, you'd at least need room for as many first-level stack frames in your internal RAM as you have concurrent processes/threads in the system, or you'd need to mutilate the context switch code to copy the first level stack frames in and out of external memory on a context switch.  I don't think that's a good path to go down.
 
If you have functioning caches, they won't be as perfect as a scratchpad, but you won't have all the additional context switch overhead, and they will automagically do approximately what you want, without your having to change any code.
 
            Kevin K.
----- Original Message -----
From: zhuzhenhua
Sent: Wednesday, September 12, 2007 4:21 AM
Subject: does the SAVE_ALL nesting in kernel?

hello, all
            i have a mips board,  and the SDRAM speed(bus clock) is not too fast.
             so i want change  the SAVE_ALL and RESTORE_ALL to use internal-ram(high speed).
            i just wonder whether the SAVE_ALL netsting in kernel  for mips arch?
            if not, i think  maybe 1k byte for SAVE_ALL is enough( 32regs X4, and some cp0_regs).
            but if  the SAVE_ALL nesting, maybe i need to keep a stack in internal-ram.
            thanks for any hints.

Best Regards

--
zzh

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