Re: [PATCH] x86/mce: Fix set_mce_nospec() to avoid #GP fault

From: Thomas Gleixner
Date: Fri Aug 31 2018 - 11:36:02 EST

On Thu, 30 Aug 2018, Linus Torvalds wrote:
> On Thu, Aug 30, 2018 at 6:49 PM Tony Luck <tony.luck@xxxxxxxxx> wrote:
> >
> > Just checking "do we have a non-canonical address" at the bottom of that
> > call stack and flipping bit 63 back on again seems like a bad idea.
> You could literally do something like
> /* Make it canonical in case we flipped the high bit */
> addr = (long)(addr<<1)>>1;
> in the call to clflush and it magically does the right thing.
> Pretty? No. But with a big comment about what is going on and why it's
> done, I think it's prettier than your much bigger patch.
> I dunno. It does strike me as a bit hacky, but I'd rather have a
> *small* one-liner hack that generates two instructions, than add a
> complex hack that modifies the page tables three times and has a
> serializing instruction in it.
> Both are subtle fixes for a subtle issue, but one seems pretty
> harmless in comparison.
> Hmm?
> But I'll bow to the x86 maintainers.

The above is fugly, but it has the charm of simplicity and I assume it's
going to be useful for other places as well. With a big fat comment WHY we
are doing it it's not that horrible. We have all the other L1TF places
where we fiddle with bits in non-obvious ways, so having another instance
of magic bit fiddling is not that big of a problem.