linux-kernel - Re: [patch V4 part 3 04/29] x86/traps: Make interrupt enable/disable symmetric in C code

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Message-ID: <87tv0rekh3.fsf@nanos.tec.linutronix.de>
Date:   Thu, 07 May 2020 19:14:00 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 3 04/29] x86/traps: Make interrupt enable/disable symmetric in C code

Alexandre Chartre <alexandre.chartre@...cle.com> writes:

> On 5/5/20 3:43 PM, Thomas Gleixner wrote:
>> Traps enable interrupts conditionally but rely on the ASM return code to
>> disable them again. That results in redundant interrupt disable and trace
>> calls.
>> 
>> Make the trap handlers disable interrupts before returning to avoid that,
>> which allows simplification of the ASM entry code.
>> 
>> Originally-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> 
>> ---
>>   arch/x86/kernel/traps.c |   28 +++++++++++++++++++---------
>>   arch/x86/mm/fault.c     |   15 +++++++++++++--
>>   2 files changed, 32 insertions(+), 11 deletions(-)
>> 
>
> So this patch makes C trap handlers disable interrupts on return but there's no
> change to the ASM entry code, which will still (also) disable interrupts. I suppose
> this is cleaned up in a next patch. So it's worth mentioning that the "simplification
> of the ASM entry code" is not in this patch.

I thought that was expressed by:

>> which allows simplification of the ASM entry code.

but yeah it's ambigous. Will clarify.

Thanks,

        tglx