Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[David Woodhouse <dwmw2@infradead.org>]

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On Sun, 2018-01-21 at 13:35 -0800, Linus Torvalds wrote:
> On Sun, Jan 21, 2018 at 12:28 PM, David Woodhouse  wrote:
> > As a hack for existing CPUs, it's just about tolerable — as long as it
> > can die entirely by the next generation.
>
> That's part of the big problem here. The speculation control cpuid
> stuff shows that Intel actually seems to plan on doing the right thing
> for meltdown (the main question being _when_). Which is not a huge
> surprise, since it should be easy to fix, and it's a really honking
> big hole to drive through. Not doing the right thing for meltdown
> would be completely unacceptable.
> 
> So the IBRS garbage implies that Intel is _not_ planning on doing the
> right thing for the indirect branch speculation.
> 
> Honestly, that's completely unacceptable too.

Agreed. I've been saying that since I first saw the IBRS_ALL proposal.
There's *no* good reason for it to be opt-in. Just fix it!

> > So the part is I think is odd is the IBRS_ALL feature, where a future
> > CPU will advertise "I am able to be not broken" and then you have to
> > set the IBRS bit once at boot time to *ask* it not to be broken. That
> > part is weird, because it ought to have been treated like the RDCL_NO
> > bit — just "you don't have to worry any more, it got better".
>
> It's not "weird" at all. It's very much part of the whole "this is
> complete garbage" issue.
> 
> The whole IBRS_ALL feature to me very clearly says "Intel is not
> serious about this, we'll have a ugly hack that will be so expensive
> that we don't want to enable it by default, because that would look
> bad in benchmarks".
> 
> So instead they try to push the garbage down to us. And they are doing
> it entirely wrong, even from a technical standpoint.

Right. The whole IBRS/IBPB thing as a nasty hack in the short term I
could live with, but it's the long-term implications of IBRS_ALL that
I'm unhappy about.

My understanding was that the IBRS_ALL performance was supposed to not
suck — to the extent that we'd just turn it on and then ALTERNATIVE out
the retpolines, and that would be the best option.

But if that's the case, why are they making it an option, and not just
doing the same as RDCL_NO does for "we fixed Meltdown"?

> > We do need the IBPB feature to complete the protection that retpoline
> > gives us — it's that or rebuild all of userspace with retpoline.
>
> BULLSHIT.
> 
> Have you _looked_ at the patches you are talking about?  You should
> have - several of them bear your name.
> 
> The patches do things like add the garbage MSR writes to the kernel
> entry/exit points. That's insane. That says "we're trying to protect
> the kernel".  We already have retpoline there, with less overhead.

You're looking at IBRS usage, not IBPB. They are different things.

Yes, the one you're looking at really *is* trying to protect the
kernel, and you're right that it's largely redundant with retpoline.
(Assuming we can live with the implications on Skylake, as I said.)

> If this was about flushing the BTB at actual context switches between
> different users, I'd believe you. But that's not at all what the
> patches do.

That's what the *IBPB* patches do. Those were deliberately put first in
the series (and in fact that's where I stopped, when I posted).

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