linux-kernel - Re: [PATCH v4 05/27] x86: Replace ist_enter() with nmi

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Date:   Mon, 24 Feb 2020 11:43:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Greg KH <gregkh@...uxfoundation.org>, gustavo@...eddedor.com,
        Thomas Gleixner <tglx@...utronix.de>, paulmck@...nel.org,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v4 05/27] x86: Replace ist_enter() with nmi_enter()

On Fri, Feb 21, 2020 at 09:22:46PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 21, 2020 at 11:05:36AM -0800, Andy Lutomirski wrote:
> 
> > > -       /*
> > > -        * Use ist_enter despite the fact that we don't use an IST stack.
> > > -        * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
> > > -        * mode or even during context tracking state changes.
> > > -        *
> > > -        * This means that we can't schedule.  That's okay.
> > > -        */
> > > -       ist_enter(regs);
> > > +       nmi_enter();
> > 
> > I agree with the change, but some commentary might be nice.  Maybe
> > copy from here:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/idtentry&id=061eaa900b4f63601ab6381ab431fcef8dfd84be
> 
> Fair enough; I'll add something to #DB and #BP for that.

do_int3() is now like:

@@ -529,19 +497,18 @@ dotraplinkage void do_general_protection
 }
 NOKPROBE_SYMBOL(do_general_protection);
 
-dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
+dotraplinkage notrace void do_int3(struct pt_regs *regs, long error_code)
 {
 	if (poke_int3_handler(regs))
 		return;
 
 	/*
-	 * Use ist_enter despite the fact that we don't use an IST stack.
-	 * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
-	 * mode or even during context tracking state changes.
-	 *
-	 * This means that we can't schedule.  That's okay.
+	 * Unlike any other non-IST entry, we can be called from pretty much
+	 * any location in the kernel through kprobes -- text_poke() will most
+	 * likely be handled by poke_int3_handler() above. This means this
+	 * handler is effectively NMI-like.
 	 */
-	ist_enter(regs);
+	nmi_enter();
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
 	if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
@@ -563,7 +530,7 @@ dotraplinkage void notrace do_int3(struc
 	cond_local_irq_disable(regs);
 
 exit:
-	ist_exit(regs);
+	nmi_exit();
 }
 NOKPROBE_SYMBOL(do_int3);
 

And I'm going to have to do another (few) patches for do_debug(),
because that comment it has it patently false and I remember we had a
bunch of crud for that. Let me dig those out.