Re: [PATCH] i386: PAE entries must have their low word cleared first

[Sonny Rao <sonny@burdell.org>]

On Thu, Apr 27, 2006 at 01:58:42AM +1000, Nick Piggin wrote:
> Keir Fraser wrote:
> >
> >On 26 Apr 2006, at 15:46, Hugh Dickins wrote:
> >
> >>If that's so (I don't trust my judgement on matters of speculative
> >>execution), then I think you'd do better to replace the *ptep = __pte(0)
> >>by pte_clear(mm, addr, ptep), and so avoid your ugly #ifdef'ing: please
> >>check, but I think you'll find that reduces to just the barrier you want.
> >>CC'ed Zach since it's his optimization, and he'll judge that spexecution.
> >
> >
> >In more detail the problem is that, since we're still running on the 
> >page tables while clearing them, the CPU may choose to prefetch a 
> >half-cleared pte into its TLB, and then execute speculative memory 
> >accesses based on that mapping (including ones that may write-allocate 
> >cachelines, leading to problems like the AMD AGP GART deadlock Linux had 
> >a year or so back).
> 
> What do you mean, you're still running on the page tables? The CPU can
> still walk the pagetables?
> 
> Because if ptep_get_and_clear_full is passed non zero in the full
> argument, then that specific translation should never see another
> access. I didn't know CPUs now actually resolve TLB misses as part of
> speculative prefetching... does this really happen?

For instance, during speculative execution on POWER, we can take a
TLB miss for a speculative load and start a table-walk.

I'm not sure what "speculative prefetching" means in this case... just
regular hardware-initiated prefetching (where I suppose one could use the
modifier "speculative") on POWER will only prefetch to a page-boundary.

So, slightly OT, as this is not about x86 CPUs... but thought people
might be interested.

(Added Anton to CC to call any BS on my part :-)

Sonny
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