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

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Message-ID: <CALCETrWS8+W3Wno=-QonePLkzeXLJ+sLt2ocxXF7QJmE_t9fWQ@mail.gmail.com>
Date:	Fri, 24 Jul 2015 21:28:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
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 Fri, Jul 24, 2015 at 9:16 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Jul 24, 2015 at 11:02:51AM -0700, Andy Lutomirski wrote:
>> So really the only difference between this simple approach (which is
>> more or less what we do now) and my fancy approach is that a kernel
>> instruction breakpoint will cause do_debug to run on the initial stack
>> instead of the IRQ stack.
>
> Sounds ok to me. What would be the worst thing if we limited the #DB
> stack? Some breakpoints will get ignored? In an endless stream of
> breakpoints hammering? Doesn't sound like a valid use case to me, does
> it?
>
>> I'm still tempted to say we should use my overly paranoid atomic
>> approach for now and optimize later,...
>
> But why change it if the simple approach of incrementing irq_count first
> is still fine? I think we want to KISS here exactly because apparently
> complexity in that area is a serious PITA...

Yeah, I'm going to submit v2 with the simple approach.  I admit I'm
rather fond of xadd as a way to switch rsp and set a flag at the same
time, though :)

--Andy
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