linux-kernel - Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C for simple idtentries

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Date:   Wed, 26 Feb 2020 18:05:34 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C
 for simple idtentries

On Wed, Feb 26, 2020 at 09:05:38AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 25, 2020 at 11:33:34PM +0100, Thomas Gleixner wrote:
> 
> > --- a/arch/x86/include/asm/idtentry.h
> > +++ b/arch/x86/include/asm/idtentry.h
> > @@ -7,14 +7,31 @@
> >  
> >  #ifndef __ASSEMBLY__
> >  
> > +#ifdef CONFIG_CONTEXT_TRACKING
> > +static __always_inline void enter_from_user_context(void)
> > +{
> > +	CT_WARN_ON(ct_state() != CONTEXT_USER);
> > +	user_exit_irqoff();
> > +}
> > +#else
> > +static __always_inline void enter_from_user_context(void) { }
> > +#endif
> > +
> >  /**
> >   * idtentry_enter - Handle state tracking on idtentry
> >   * @regs:	Pointer to pt_regs of interrupted context
> >   *
> > - * Place holder for now.
> > + * Invokes:
> > + *  - The hardirq tracer to keep the state consistent as low level ASM
> > + *    entry disabled interrupts.
> > + *
> > + *  - Context tracking if the exception hit user mode
> >   */
> >  static __always_inline void idtentry_enter(struct pt_regs *regs)
> >  {
> > +	trace_hardirqs_off();
> > +	if (user_mode(regs))
> > +		enter_from_user_context();
> >  }
> 
> So:
> 
> 	asm_exc_int3
> 	  exc_int3
> 	    idtentry_enter()
> 	      enter_from_user_context
> 	        if (context_tracking_enabled())
> 
> 	    poke_int3_handler();
> 
> Is, AFAICT, completely buggered.
> 
> You can't have a static_branch before the poke_int3_handler that deals
> with text_poke.

#BP is treated like an NMI in your patchset IIRC?
In that case and since that implies we can't schedule, we can remove
the call to context tracking there once we have nmi_enter()/nmi_exit()
called unconditionally.