Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

Avi Kivity
2010-07-16T16:48:08Z

On 07/16/2010 05:49 PM, Mathieu Desnoyers wrote:
>
>> You need to save/restore cr2 in addition, otherwise the following hits you
>>
>> - page fault
>> - processor writes cr2, enters fault handler
>> - nmi
>> - page fault
>> - cr2 overwritten
>>
>> I guess you would usually not notice the corruption since you'd just see
>> a spurious fault on the page the NMI handler touched, but if the first
>> fault happened in a kvm guest, then we'd corrupt the guest's cr2.
>>
> OK, just to make sure: you mean we'd have to save/restore the cr2 register
> at the beginning/end of the NMI handler execution, right ?

Yes.

> The shouldn't we
> save/restore cr3 too ?
>
>

No, faults should not change cr3.

>> But the whole thing strikes me as overkill. If it's 8k per-cpu, what's
>> wrong with using a per-cpu pointer to a kmalloc() area?
>>
> Well, it seems like all the kernel code calling "vmalloc_sync_all()" (which is
> much more than perf) can potentially cause large latencies, which could be
> squashed by allowing page faults in NMI handlers. This looks like a stronger
> argument to me.

Why is that kernel code calling vmalloc_sync_all()? If it is only NMI
which cannot take vmalloc faults, why bother? If not, why not?

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.