svn commit: r210846 - in head/sys/mips: include mips

[Jayachandran C.]

On Fri, Aug 6, 2010 at 10:06 PM, Alan Cox <alc at cs.rice.edu> wrote:
> The patch looks good.
>
> While we're talking about software dirty bit emulation, I would encourage
> you to look at two things:
>
> 1. trap.c contains two copies of the same code for emulation.  I would
> encourage you to eliminate this duplication by creating a
> pmap_emulate_modified().
>
> 2. Software dirty bit emulation is using pmap_update_page() to invalidate
> the TLB entry on which the modified bit is being set.  On a multiprocessor,
> this is going to make dirty bit emulation very costly because every
> processor will be interrupted.  In principle, it should be possible and
> faster to only flush the TLB entry from the current processor.  The other
> processors can handle this lazily.  They either do not have that mapping in
> their TLB, in which case interrupting them was wasted effort, or they do
> have it in their TLB and when they fault on it they'll discover the dirty
> bit is already set.  In fact, the emulation code already handles this case,
> on account of the fact that two processors could simultaneously write to the
> same clean page and only one will get the pmap lock first.

I've made the changes suggested, the changes are attached.

The first set of changes just re-arranges the pmap calls that use
smp_rendezvous() on SMP, so that their per-cpu variants are also
available to be called.  The first patch also has an optimization from
Juli's branch, to call pmap_update_page in pmap_kenter only if the pte
is valid.

The second patch makes the changes suggested above. My testing shows
no issues so far, but please let me know if you have any comments.

Thanks,
JC.
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