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Shadow registers optimizations

To: linux-mips@linux-mips.org
Subject: Shadow registers optimizations
From: Nikolaos Korkakakis <nikolaos.korkakakis@sitelsemi.com>
Date: Mon, 28 Feb 2011 10:15:04 +0200
Cc: ralf@linux-mips.org, korkakak@gmail.com
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Good morning list,

I would like to do some optimizations in the kernel to boost even more the performance of the m14kc (and potentially m4 series as well). One of the interesting ideas that has the potential in a busy system to provide higher performance are the shadow registers for interrupt handling code (maybe there are other usages for shadow registers in other areas by I leave this aside).

I had a somewhat long talk with Ralf over irc last friday, which started from the point that in kernel/traps.c there are two vi handler installers. A helper (set_vi_srs_handler) and a wrapper of the helper (set_vi_handler)

the helper in case of shadow sets exists it only adds a jump in the vi table (the "0x08000000 | (((u32)handler >> 2) & 0x03fffff);" is that kind of magic) followed by a nop. that's all. that's ok for a helper but it may have a performance impact since on each dump the whole execution pipeline is flushed, not to mention the actual loss of the whole VECTORSPACING (which is hard set to 0x100 bytes) memory space for just 8bytes of code.

So I intend to change some things around the traps code.
- The first is how the VECTORSPACING is defined (no constant declaration anymore) it is gonna be a dynamic init based setup that would alter the intctl register as it ought to in the first place - the second is that in case of shadow register != 0 the handlers (ie the plat_irq_dispatch) should be copied there if the VECTORSPACING space is sufficient or else use the jump install code

loose ends. Ralf mentioned that the usage of shadow registers is not str8 fwd and a careful approach should be taken. those are
 1) arch/mips/kernel/entry.S  _TIF_NEED_RESCHED
 2) asm/irq_regs.h get_irq_regs() [rdpgpr and wrpgpr]
 3) <add yours>

For 1 and 2 the solution seems pretty trivial, but if you have any other ideas please elaborate on this so the loose ends are pinpointed and attacked.

BR,
N

--
Nikolaos Korkakakis
Embedded Software Engineer
Tel:         +30 2610 390 948
OS and Applications Group
SiTel Semiconductor BV

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