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

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Date:   Mon, 19 Feb 2018 10:20:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>, hpa@...or.com,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware

On Sat, Feb 17, 2018 at 11:26:16AM +0100, Ingo Molnar wrote:
> Note that PeterZ was struggling with intermittent boot hangs yesterday as well, 
> which hangs came and went during severeal (fruitless) bisection attempts. Then at 
> a certain point the hangs went away altogether.
> 
> The symptoms for both his and your hangs are consistent with an alignment 
> dependent bug.

Mine would consistently hang right after

  "Freeing SMP alternatives memory: 44K"

At one point I bisected it to commit:

  a06cc94f3f8d ("x86/build: Drop superfluous ALIGN from the linker script")

But shortly thereafter I started having trouble reproducing, and now I
can run kernels that before would consistently fail to boot :/

> My other guess is that it's perhaps somehow microcode related?

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

Like I wrote on IRC; what _seems_ to have 'cured' things is clearing out
my /boot. The amount of kernels generated by the bisect was immense and
running 'update-grub' was taking _much_ longer than actually building a
new kernel.

What I have not tried is again generating and installing that many
kernels to see if that will make it go 'funny' again.