linux-kernel - Re: [tip:x86/pti] Revert "x86/speculation: Simplify indirect_branch_prediction

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Date:   Tue, 13 Feb 2018 12:28:38 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, dave.hansen@...ux.intel.com,
        bp@...en8.de, tglx@...utronix.de, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org, dwmw2@...radead.org, hpa@...or.com,
        gregkh@...uxfoundation.org, dan.j.williams@...el.com,
        dwmw@...zon.co.uk, luto@...nel.org, jpoimboe@...hat.com,
        linux-tip-commits@...r.kernel.org,
        Joe Konno <joe.konno@...ux.intel.com>
Subject: Re: [tip:x86/pti] Revert "x86/speculation: Simplify
 indirect_branch_prediction_barrier()"


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

> 
> On Tue, Feb 13, 2018 at 12:58:21AM -0800, tip-bot for David Woodhouse wrote:
> 
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -164,10 +164,15 @@ static inline void vmexit_fill_RSB(void)
> >  
> >  static inline void indirect_branch_prediction_barrier(void)
> >  {
> > +	asm volatile(ALTERNATIVE("",
> > +				 "movl %[msr], %%ecx\n\t"
> > +				 "movl %[val], %%eax\n\t"
> > +				 "movl $0, %%edx\n\t"
> > +				 "wrmsr",
> > +				 X86_FEATURE_USE_IBPB)
> > +		     : : [msr] "i" (MSR_IA32_PRED_CMD),
> > +			 [val] "i" (PRED_CMD_IBPB)
> > +		     : "eax", "ecx", "edx", "memory");
> >  }
> 
> Joe Konno pointed out that we now need the below line too, because we're
> using MSR_IA32_PRED_CMD in this header.
> 
> With the existing code that's not a problem per-se, but my objtool
> retpoline annotation things did do stumble over this.
> 
> Do we want to fold it into the objtool annotation patch or have it
> separate?

Separate would be better, it makes sense and is one problem less to worry about?

Thanks,

	Ingo