linux-kernel - Re: [PATCH v3 03/22] x86: Replace ist_enter() with nmi

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Date:   Thu, 20 Feb 2020 13:11:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        luto@...nel.org, tony.luck@...el.com, frederic@...nel.org,
        dan.carpenter@...cle.com, mhiramat@...nel.org
Subject: Re: [PATCH v3 03/22] x86: Replace ist_enter() with nmi_enter()

On Thu, Feb 20, 2020 at 11:54:39AM +0100, Borislav Petkov wrote:
> On Wed, Feb 19, 2020 at 03:47:27PM +0100, Peter Zijlstra wrote:
> > @@ -1220,7 +1220,7 @@ static void mce_kill_me_maybe(struct cal
> >   * MCE broadcast. However some CPUs might be broken beyond repair,
> >   * so be always careful when synchronizing with others.
> >   */
> > -void do_machine_check(struct pt_regs *regs, long error_code)
> > +notrace void do_machine_check(struct pt_regs *regs, long error_code)
> 
> Is there a convention where the notrace marker should come in the
> function signature? I see all possible combinations while grepping...

Same place as inline I think.

> >  {
> >  	DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
> >  	DECLARE_BITMAP(toclear, MAX_NR_BANKS);
> > @@ -1254,10 +1254,10 @@ void do_machine_check(struct pt_regs *re
> >  	 */
> >  	int lmce = 1;
> >  
> > -	if (__mc_check_crashing_cpu(cpu))
> > -		return;
> > +	nmi_enter();
> >  
> > -	ist_enter(regs);
> > +	if (__mc_check_crashing_cpu(cpu))
> > +		goto out;
> >  
> >  	this_cpu_inc(mce_exception_count);
> >  
> 
> Should that __mc_check_crashing_cpu() happen before nmi_enter? The
> function is doing only a bunch of checks and clearing MSRs for bystander
> CPUs...

You'll note the lack of notrace on that function, and we must not call
into tracers before nmi_enter().

AFAICT there really is no benefit to trying to lift it before
nmi_enter().