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Re: [Xen-devel] [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Mar 14, 2016 at 10:17 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> > So yes, let's please warn.  I'm okay with removing the panic_on_oops
> > thing though.  (But if anyone suggests that we should stop OOPSing on
> > bad kernel page faults, I *will* fight back.)
> Bad kernel page faults are something completely different. They would
> be actual bugs regardless.
> The MSR thing has *often* been just silly "this CPU doesn't do that
> MSR". Generic bootup setup code etc that just didn't know or care
> about the particular badly documented rule for one particular random
> CPU version and stepping.
> In fact, when I say "often", I suspect I should really just say
> "always". I don't think we've ever found a case where oopsing would
> have been the right thing. But it has definitely caused lots of
> problems, especially in the early paths where your code doesn't even
> work right now.
> Now, when it comes to the warning, I guess I could live with it, but I
> think it's stupid to make this a low-level exception handler thing.
> So what I think should be done:
>  - make sure that wr/rdmsr_safe() actually works during very early
> init. At some point it didn't.
>  - get rid of the current wrmsr/rdmsr *entirely*. It's shit.
>  - Add this wrapper:
>       #define wrmsr(msr, low, high) \
>         WARN_ON_ONCE(wrmsr_safe(msr, low, high))
> and be done with it. We could even decide to make that WARN_ON_ONCE()
> be something we could configure out, because it's really a debugging
> thing and isn't like it should be all that fatal.
> None of this insane complicated crap that buys us exactly *nothing*,
> and depends on fancy new exception handling support etc etc.
> So what's the downside to just doing this simple thing?

This was my original suggestion as well.

The objection was that sometimes it's performance critical. To which I replied 
that 1) I highly doubt that a single extra branch of the check is measurable 2) 
and even if so, then _those_ performance critical cases should be done in some 
special way (rdmsr_nocheck() or whatever) - at which point the argument was 
there's a lot of such cases.

Which final line of argument I still don't really buy, btw., but it became a me 
against everyone else argument and I cowardly punted.

Now that the 800 lb gorilla is on my side I of course stand my ground, 
are principles!



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