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

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Date:   Fri, 16 Feb 2018 19:16:40 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        hpa@...or.com, tglx@...utronix.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware

On Fri, 2018-02-16 at 10:44 -0800, Tim Chen wrote:
> 
> I encountered hang on a machine but not others when using the above
> macro.  It is probably an alignment thing with ALTERNATIVE as the
> problem went
> away after I made the change below:
> 
> Tim
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h
> b/arch/x86/include/asm/nospec-branch.h
> index 8f2ff74..0f65bd2 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -148,6 +148,7 @@ extern char __indirect_thunk_end[];
>  
>  #define alternative_msr_write(_msr, _val, _feature)            \
>         asm volatile(ALTERNATIVE("",                            \
> +                               ".align 16\n\t"                \
>                                 "movl %[msr], %%ecx\n\t"       \
>                                 "movl %[val], %%eax\n\t"       \
>                                 "movl $0, %%edx\n\t"           \

That's weird. Note that .align in an altinstr section isn't actually
going to do what you'd expect; the oldinstr and altinstr sections
aren't necessarily aligned the same, so however many NOPs it inserts
into the alternative, might be deliberately *misaligning* it in the
code that actually gets executed.

Are you sure you're not running a kernel where the alternatives code
would turn that alternative which *starts* with a NOP, into *all* NOPs?
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