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

[Borislav Petkov]

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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