Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

David Woodhouse
2018-02-19T09:44:29Z

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On Mon, 2018-02-19 at 10:39 +0100, Ingo Molnar wrote:
> * David Woodhouse <dwmw2@infradead.org> wrote:
>
> >
> > On Mon, 2018-02-19 at 10:20 +0100, Peter Zijlstra wrote:
> > >
> > >
> > > I did not update or otherwise change packages while I was bisecting; the
> > > machine is:
> > >
> > > vendor_id       : GenuineIntel
> > > cpu family      : 6
> > > model           : 62
> > > model name      : Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz
> > > stepping        : 4
> > > microcode       : 0x428
> > That's IVX with a microcode that doesn't *have* IBRS/IBPB. I don't
> > think there's a publicly available microcode that does; I assume you
> > didn't have one and build it into your kernel for early loading, and
> > thus you really weren't even using IBRS here? The code never even gets
> > patched in?
> Note that PeterZ's boot troubles only match the *symptoms* of the spurious
> failures reported by Tim Chen. Your commit wasn't bisected to.
>
> I linked these two reports on the (remote) possibility that they might be related
> via some alignment dependent bug somewhere else in the x86 kernel - possibly
> completely unrelated to any IBRS/IBPB details.

Understood. I was merely *accentuating* the "completely unrelated to
any IBRS/IBPB" bit, in a "la la la I'm ignoring this because I'm
working on other things" kind of a way... :)

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