Re: increased translation cache footprint in v2.6

["David S. Miller" <davem@davemloft.net>]

From: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Date: Sun, 26 Jun 2005 16:09:44 -0300

> Thats a very interesting idea, will probably optimize performance in 
> general ("why did nobody thought of it before?" kind). 
> 
> The increase in TLB miss handler size might be offset by the reduced
> kernel misses...
> 
> Dan, what do you think? 

I doubt it, it cost a single comparison on sparc64 to implement this.

Basically, the TLB miss handler for data accesses on sparc64 looks
like the following.

Load the miss information:

	ldxa		[%g1 + %g1] ASI_DMMU, %g4	! Get TAG_ACCESS

If "TAG_CONTEXT_BITS" is zero, it's for the kernel:

	andcc		%g4, TAG_CONTEXT_BITS, %g0	! From Nucleus?
	be,pn		%xcc, 3f			! Yep, special processing
 ...

If the virtual address has the top-most bit set (and thus it's
"negative"), then it's in the physical memory direct mapping
area.  The trap handler register %g2 is preloaded with a fixed
value, that when XOR'd with the fault address information
produces a suitable PTE for loading right into the TLB.  This
PTE uses 4MB pages.

3:	brlz,pt		%g4, 9b				! Kernel virtual map?
	 xor		%g2, %g4, %g5			! Finish bit twiddles

 ...

Store the calculated TLB entry and return from the trap.

9:	stxa		%g5, [%g0] ASI_DTLB_DATA_IN	! Reload TLB
	retry						! Trap return

So that's 7 instructions, 2 instruction cache lines, with no main
memory accesses.  Surely the PPC folks can do something similar. :-)