Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption

[Linus Torvalds]

On Fri, Jul 5, 2019 at 6:50 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Also; all previous attempts at fixing this have been about pushing the
> read_cr2() earlier; notably:
>
>   0ac09f9f8cd1 ("x86, trace: Fix CR2 corruption when tracing page faults")
>   d4078e232267 ("x86, trace: Further robustify CR2 handling vs tracing")

I think both of those are because people - again - felt like tracing
can validly corrupt CPU state, and then they fix up the symptoms
rather than the cause.

Which I disagree with.

> And I'm thinking that with exception of this patch, the rest are
> worthwhile cleanups regardless.

I don't have any issues with the patches themselves, I agree that they
are probably good on their own.

I *do* have issues with the "tracing can change CPU state so we need
to deal with it" model, though. I think that mode of thinking is
wrong. I don't believe tracing should ever change core CPU state and
that be considered ok.

> Also; while looking at this, if we do continue with the C wrappers from
> the very last patch, we can do horrible things like this on top and move
> the read_cr2() back into C code.

Again, I don't think that is the problem. I think it's a much more
fundamental problem in thinking that core code should be changed
because tracing is broken garbage and didn't do things right.

I see Eiichi's patch, and it makes me go "that looks better" - simply
because it fixes the fundamental issue, rather than working around the
symptoms.

               Linus