linux-kernel - Re: [patch V5 06/38] x86/entry: Provide helpers for execute on irqstack

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Message-ID: <20200513214300.ruikp7ldnko7t5pi@treble>
Date:   Wed, 13 May 2020 16:43:00 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, 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>,
        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 V5 06/38] x86/entry: Provide helpers for execute on
 irqstack

On Tue, May 12, 2020 at 11:01:05PM +0200, Thomas Gleixner wrote:
> Device interrupt handlers and system vector handlers are executed on the
> interrupt stack. The stack switch happens in the low level assembly entry
> code. This conflicts with the efforts to consolidate the exit code in C to
> ensure correctness vs. RCU and tracing.
> 
> As there is no way to move #DB away from IST due to the MOV SS issue, the
> requirements vs. #DB and NMI for switching to the interrupt stack do not
> exist anymore. The only requirement is that interrupts are disabled.
> 
> That allows to move the stack switching to C code which simplifies the
> entry/exit handling further because it allows to switch stacks after
> handling the entry and on exit before handling RCU, return to usermode and
> kernel preemption in the same way as for regular exceptions.
> 
> The initial attempt of having the stack switching in inline ASM caused too
> much headache vs. objtool and the unwinder. After analysing the use cases
> it was agreed on that having the stack switch in ASM for the price of an
> indirect call is acceptable as the main users are indirect call heavy
> anyway and the few system vectors which are empty shells (scheduler IPI and
> KVM posted interrupt vectors) can run from the regular stack.
> 
> Provide helper functions to check whether the interrupt stack is already
> active and whether stack switching is required.
> 
> 64 bit only for now. 32 bit has a variant of that already. Once this is
> cleaned up the two implementations might be consolidated as a cleanup on
> top.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lore.kernel.org/r/20200507161020.783541450@infradead.org
> ---
> V5: Moved the actual switch to ASM code

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh