linux-kernel - Re: [patch V2 08/17] x86/entry: Move syscall irq tracing to C code

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Message-ID: <alpine.DEB.2.21.1910240124410.1852@nanos.tec.linutronix.de>
Date:   Thu, 24 Oct 2019 01:31:38 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [patch V2 08/17] x86/entry: Move syscall irq tracing to C code

On Wed, 23 Oct 2019, Andy Lutomirski wrote:
> On Wed, Oct 23, 2019 at 2:30 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > On Wed, Oct 23, 2019 at 5:31 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > >
> > > Interrupt state tracing can be safely done in C code. The few stack
> > > operations in assembly do not need to be covered.
> > >
> > > Remove the now pointless indirection via .Lsyscall_32_done and jump to
> > > swapgs_restore_regs_and_return_to_usermode directly.
> >
> > This doesn't look right.
> >
> > >  #define SYSCALL_EXIT_WORK_FLAGS                                \
> > > @@ -279,6 +282,9 @@ static void syscall_slow_exit_work(struc
> > >  {
> > >         struct thread_info *ti;
> > >
> > > +       /* User to kernel transition disabled interrupts. */
> > > +       trace_hardirqs_off();
> > > +
> >
> > So you just traced IRQs off, but...
> >
> > >         enter_from_user_mode();
> > >         local_irq_enable();
> >
> > Now they're on and traced on again?
> >
> > I also don't see how your patch handles the fastpath case.
> >
> > How about the attached patch instead?
> 
> Ignore the attached patch.  You have this in your
> do_exit_to_usermode() later in the series.  But I'm still quite
> confused by this patch.

What's confusing you? It basically does:

  ENTRY(syscall/int80)

-	TRACE_IRQS_OFF
	call C-syscall*()
-	TRACE_IRQS_ON/IRET

and

C-syscall*()

+       trace_hardirqs_off()		<- first action
	....
	prepare_exit_to_usermode()	<- last action
	return

and

prepare_exit_to_usermode()
	....
+       trace_hardirqs_on()		<- last action
	return

So this is exactly the same as the ASM today.

The only change is that I made it do unconditionally trace_hardirqs_on()
for consistency reasons.

I tried to split it into bits and pieces, but failed to come up with
something sensible. Let me try again tomorrow.

Thanks,

	tglx