Re: [PATCH 2/3] x86,mm: drop TLB flush from ptep_set_access_flags

On Sat, Nov 17, 2012 at 7:24 AM, Rik van Riel [off-list ref] wrote:

On 11/17/2012 09:56 AM, Linus Torvalds wrote:

quoted

On Sat, Nov 17, 2012 at 6:50 AM, Borislav Petkov [off-list ref] wrote:

quoted

I don't know, however, whether it would be prudent to have some sort of
a cheap assertion in the code (cheaper than INVLPG %ADDR, although on
older cpus we do MOV CR3) just in case. This should be enabled only with
DEBUG_VM on, of course...

I wonder how we could actually test for it. We'd have to have some
per-cpu page-fault address check (along with a generation count on the
mm or similar). I doubt we'd figure out anything that works reliably
and efficiently and would actually show any problems

Would it be enough to simply print out a warning if we fault
on the same address twice (or three times) in a row, and then
flush the local TLB?

I realize this would not just trigger on CPUs that fail to
invalidate TLB entries that cause faults, but also on kernel
paths that cause a page fault to be re-taken...

I'm actually curious if the architecture docs/software developer
manuals for IA-32 mandate any TLB invalidations on a #PF

Is there any official vendor documentation on the subject?

And perhaps equally valid, should we trust it if it exists?

... but then again, don't we want to find those paths and
fix them, anyway? :)

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