linux-kernel - Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Date:   Wed, 30 Jun 2021 13:47:08 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     jpoimboe@...hat.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, joro@...tes.org,
        boris.ostrovsky@...cle.com, jgross@...e.com, x86@...nel.org,
        mbenes@...e.com, rostedt@...dmis.org, dvyukov@...gle.com,
        elver@...gle.com
Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as
 dead_end_function

On Thu, 24 Jun 2021, Peter Zijlstra wrote:

> The asm_cpu_bringup_and_idle() function is required to push the return
> value on the stack in order to make ORC happy, but the only reason
> objtool doesn't complain is because of a happy accident.
> 
> The thing is that asm_cpu_bringup_and_idle() doesn't return, so
> validate_branch() never terminates and falls through to the next
> function, which in the normal case is the hypercall_page. And that, as
> it happens, is 4095 NOPs and a RET.
> 
> Make asm_cpu_bringup_and_idle() terminate on it's own, by making the
> function it calls as a dead-end. This way we no longer rely on what
> code happens to come after.
> 
> Fixes: c3881eb58d56 ("x86/xen: Make the secondary CPU idle tasks reliable")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Reviewed-by: Miroslav Benes <mbenes@...e.cz>

M