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Date:   Wed, 26 Feb 2020 09:05:38 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Steven Rostedt <rostedt@goodmis.org>,
        Brian Gerst <brgerst@gmail.com>,
        Juergen Gross <jgross@suse.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C
 for simple idtentries
Message-ID: <20200226080538.GO18400@hirez.programming.kicks-ass.net>
References: <20200225223321.231477305@linutronix.de>
 <20200225224145.444611199@linutronix.de>
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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.