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Date: Sat, 25 Jul 2015 11:12:29 -0700
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Subject: Re: [PATCH 1/3] x86/entry/64: Refactor IRQ stacks and make then NMI-safe
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>, Steven Rostedt <rostedt@goodmis.org>,
        Willy Tarreau <w@1wt.eu>, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
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On Sat, Jul 25, 2015 at 10:59 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> What if we added something like:
>
> if (regs->ip == ret_after_sti && !user_mode(regs) && (regs->flags &
> X86_EFLAGS_IF)) {
>   regs->ip--;
>   regs->flags &= ~X86_EFLAGS_IF;
> }
>
> to do_nmi, do_machine_check, and do_debug (the latter because kernel
> breakpoints, damnit)?

Hmm. And how would you test it?

Putting an instruction breakpoint on the final 'ret' might do it, I
guess. "mov ss" disables even that (and is documented to disable nmi
too), but maybe it works with 'sti; ret'.

But yes, as long as we'd have some test coverage, that sounds doable.

                Linus