Re: [patch 1/2] x86_64 page fault NMI-safe

[Linus Torvalds]

On Thu, Jul 15, 2010 at 3:01 PM, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
>                 . NMI exit code
> and fake NMI entry are made reentrant with respect to NMI handler interruption
> by testing, at the very beginning of the NMI handler, if a NMI is nested over
> the whole nmi_atomic .. nmi_atomic_end code region.

That is totally bogus. The NMI can be nested by exceptions and
function calls - the whole _point_ of this thing. So testing "rip" for
anything else than the specific final "iret" is meaningless. You will
be in an NMI region regardless of what rip is.

> This code assumes NMIs have a separate stack.

It also needs to be made per-cpu (and the flags be per-cpu).

Then you could in fact possibly test the stack pointer for whether it
is in the NMI stack area, and use the value of %rsp itself as the
flag. So you could avoid the flag entirely. Because testing %rsp is
valid - testing %rip is not.

That would also avoid the race, because %rsp (as a flag) now gets
cleared atomically by the "iret". So that might actually solve things.

                          Linus
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