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 10:36:35 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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


* Peter Zijlstra <peterz@...radead.org> wrote:

> 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")

So, just to make sure this commit had no effect: I cannot really see anything 
wrong with that commit, it does a single substantial change, which is to remove 
this explicit alignment:

                . = ALIGN(8);
                TEXT_TEXT

which seems fine to me, since it expanded to:

                . = ALIGN(8);
                ALIGN_FUNCTION();                                       \
		...

which expanded to:

                . = ALIGN(8);
		. = ALIGN(8);
		...

... which duplication the commit removed.

... where all the relevant defitions of TEXT_TEXT and ALIGN_FUNCTION are 
unconditional and not overriden anywhere for arch/x86 builds.

I.e. the commit is a NOP AFAICS.

Thanks,

	Ingo