linux-kernel - Re: [patch V6 13/37] x86/entry: Switch page fault exception to IDTENTRY

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Date:   Tue, 19 May 2020 13:12:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...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 13/37] x86/entry: Switch page fault exception to IDTENTRY_RAW

On Fri, May 15, 2020 at 5:10 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
>
> Convert page fault exceptions to IDTENTRY_RAW:
>   - Implement the C entry point with DEFINE_IDTENTRY_RAW
>   - Add the CR2 read into the exception handler
>   - Add the idtentry_enter/exit_cond_rcu() invocations in
>     in the regular page fault handler and use the regular
>     idtentry_enter/exit() for the async PF part.
>   - Emit the ASM stub with DECLARE_IDTENTRY_RAW
>   - Remove the ASM idtentry in 64bit
>   - Remove the CR2 read from 64bit
>   - Remove the open coded ASM entry code in 32bit
>   - Fixup the XEN/PV code
>   - Remove the old prototypes
>

Acked-by: Andy Lutomirski <luto@...nel.org>

although if you make the irq_enter_cond_rcu() mode unconditional, then
this comment can go away too:

> +       /*
> +        * Entry handling for valid #PF from kernel mode is slightly
> +        * different: RCU is already watching and rcu_irq_enter() must not
> +        * be invoked because a kernel fault on a user space address might
> +        * sleep.
> +        *
> +        * In case the fault hit a RCU idle region the conditional entry
> +        * code reenabled RCU to avoid subsequent wreckage which helps
> +        * debugability.
> +        */
> +       rcu_exit = idtentry_enter_cond_rcu(regs);

--Andy