linux-kernel - Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond

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Message-ID: <87v9kq4rjo.fsf@nanos.tec.linutronix.de>
Date:   Wed, 20 May 2020 16:19:55 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        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>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>
Subject: Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu()

Andy Lutomirski <luto@...nel.org> writes:
> On Tue, May 19, 2020 at 2:20 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> Unless I've missed something, the effect here is that #PF hitting in
> an RCU-watching context will skip rcu_irq_enter(), whereas all IRQs
> (because you converted them) as well as other faults and traps will
> call rcu_irq_enter().

The only reason why this is needed for #PF is that a kernel mode #PF may
sleep. And of course you cannot sleep after calling rcu_irq_enter().

All other interrupts/traps/system vectors cannot sleep ever. So it's a
straight forward enter/exit.

> Once upon a time, we did this horrible thing where, on entry from user
> mode, we would turn on interrupts while still in CONTEXT_USER, which
> means we could get an IRQ in an extended quiescent state.  This means
> that the IRQ code had to end the EQS so that IRQ handlers could use
> RCU.  But I killed this a few years ago -- x86 Linux now has a rule
> that, if IF=1, we are *not* in an EQS with the sole exception of the
> idle code.
>
> In my dream world, we would never ever get IRQs while in an EQS -- we
> would do MWAIT with IF=0 and we would exit the EQS before taking the
> interrupt.  But I guess we still need to support HLT, which means we
> have this mess.

You always can dream, but dont complain about the nightmares :)

Thanks,

        tglx