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SubjectRe: [patch 1/2] x86_64 page fault NMI-safe
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On Wed, Jul 14, 2010 at 10:06 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>>
>> This patch (1/2) doesn't look horrible per se. I have no problems with
>> it. I just want to understand why it is needed.

[ And patch 2/2 is much more intrusive, and touches a critical path
too.. If it was just the 1/2 series, I don't think I would care. For
the 2/2, I think I'd want to explore all the alternative options ]

> The problem originally addressed by this patch is the case where a NMI handler
> try to access vmalloc'd per-cpu data, which goes as follow:
>
> - One CPU does a fork(), which copies the basic kernel mappings.
> - Perf allocates percpu memory for buffer control data structures.
>  This mapping does not get copied.
> - Tracing is activated.
> - switch_to() to the newly forked process which missed the new percpu
>  allocation.
> - We take a NMI, which touches the vmalloc'd percpu memory in the Perf tracing
>  handler, therefore leading to a page fault in NMI context. Here, we might be
>  in the middle of switch_to(), where ->current might not be in sync with the
>  current cr3 register.

Ok. I was wondering why anybody would allocate core percpu variables
so late that this would ever be an issue, but I guess perf is a
reasonable such case. And reasonable to do from NMI.

That said - grr. I really wish there was some other alternative than
adding yet more complexity to the exception return path. That "iret
re-enables NMI's unconditionally" thing annoys me.

In fact, I wonder if we couldn't just do a software NMI disable
instead? Hav ea per-cpu variable (in the _core_ percpu areas that get
allocated statically) that points to the NMI stack frame, and just
make the NMI code itself do something like

NMI entry:
- load percpu NMI stack frame pointer
- if non-zero we know we're nested, and should ignore this NMI:
- we're returning to kernel mode, so return immediately by using
"popf/ret", which also keeps NMI's disabled in the hardware until the
"real" NMI iret happens.
- before the popf/iret, use the NMI stack pointer to make the NMI
return stack be invalid and cause a fault
- set the NMI stack pointer to the current stack pointer

NMI exit (not the above "immediate exit because we nested"):
clear the percpu NMI stack pointer
Just do the iret.

Now, the thing is, now the "iret" is atomic. If we had a nested NMI,
we'll take a fault, and that re-does our "delayed" NMI - and NMI's
will stay masked.

And if we didn't have a nested NMI, that iret will now unmask NMI's,
and everything is happy.

Doesn't the above sound like a good solution? In other words, we solve
the whole problem by simply _fixing_ the crazy Intel "iret-vs-NMI"
semantics. And we don't need to change the hotpath, and we'll just
_allow_ nested faults within NMI's.

Am I missing something? Maybe I'm not as clever as I think I am... But
I _feel_ clever.

Linus
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