Re: [PATCH v3 06/11] x86/mm: Rework lazy TLB mode and TLB freshness tracking

From: Borislav Petkov
Date: Thu Jun 22 2017 - 15:05:53 EST

On Thu, Jun 22, 2017 at 10:47:29AM -0700, Andy Lutomirski wrote:
> I figured that some future reader of this patch might actually want to
> see this text, though.

Oh, don't get me wrong: with commit messages more is more, in the
general case. That's why I said "if".

> >> The UV tlbflush code is rather dated and should be changed.
> And I'd definitely like the UV maintainers to notice this part, now or
> in the future :) I don't want to personally touch the UV code with a
> ten-foot pole, but it really should be updated by someone who has a
> chance of getting it right and being able to test it.

Ah, could be because they moved recently and have hpe addresses now.
Lemme add them.

> >> +
> >> + if (cpumask_test_cpu(cpu, mm_cpumask(mm)))
> >> + cpumask_clear_cpu(cpu, mm_cpumask(mm));
> >
> > It seems we haz a helper for that: cpumask_test_and_clear_cpu() which
> > does BTR straightaway.
> Yeah, but I'm doing this for performance. I think that all the
> various one-line helpers do a LOCKed op right away, and I think it's
> faster to see if we can avoid the LOCKed op by trying an ordinary read
> first.

Right, the test part of the operation is unlocked so if that is the
likely case, it is a win.

> OTOH, maybe this is misguided -- if the cacheline lives somewhere else
> and we do end up needing to update it, we'll end up first sharing it
> and then making it exclusive, which increases the amount of cache
> coherency traffic, so maybe I'm optimizing for the wrong thing. What
> do you think?

Yeah, but we'll have to do that anyway for the locked operation. Ok,
let's leave it split like it is.

> It did in one particular buggy incarnation. It would also trigger if,
> say, suspend/resume corrupts CR3. Admittedly this is unlikely, but
> I'd rather catch it. Once PCID is on, corruption seems a bit less
> farfetched -- this assertion will catch anyone who accidentally does
> write_cr3(read_cr3_pa()).

Ok, but let's put a comment over it pls as it is not obvious when
something like that can happen.


Good mailing practices for 400: avoid top-posting and trim the reply.