linux-kernel - Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then NMI-safe

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Message-ID: <20150724102503.GA19090@nazgul.tnic>
Date:	Fri, 24 Jul 2015 12:25:03 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, Willy Tarreau <w@....eu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then
 NMI-safe

On Thu, Jul 23, 2015 at 11:08:39PM -0700, Andy Lutomirski wrote:
> To be obviously safe against any local exception, we want a single
> instruction that will change %rsp and some in-memory flag at the same
> time.  There aren't a whole lot of candidates.  Cmpxchg isn't useful
> (cmpxchg with a memory operand doesn't modify its register operand).

Why would you even need that?

You do LOCK; CMPXCHG on a per_cpu variable and then test ZF? I.e., use
it as a mutex in asm. With ZF=1, you switch stacks, with ZF=0, you
busy-wait ...

Or am I missing something?

This way you serialize all irq stack switchers...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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