Re: [PATCH] KVM: SVM: fix random segfaults with NPT enabled

Avi Kivity
2008-08-27T16:59:26Z

Avi Kivity wrote:
> Joerg Rodel wrote:
>>> Meanwhile, I applied the patch, but I'm very worried about this.
>>>
>>
>> Yes, we are also worried. Another question is why this only happens with
>> NPT. The SoftMMU code should also fail with shadow paging if there is a
>> bug.
>>
>
> Slightly different paths -- direct_map vs page_fault. Also, with npt,
> all cpus will access the same pte that's being modified; without npt,
> faults on the same page will result in different ptes being
> instantiated, as each access will be from a different guest pte.
>
> Maybe we should turn on the dirty bit in the instantiated ptes -- that
> will reduce the processor's mucking about with them.
>

I meant the accessed bit. The dirty bit is always set, but the accessed
bit it not, due to a bug. Fixing it doesn't help, though.

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