Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted

Avi Kivity
2010-06-28T11:13:27Z

On 06/28/2010 01:02 PM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>
>
>> Instead of adding a new bit, can you encode the protection in the direct
>> sp's access bits? So we'll have one sp for read-only or
>> writeable-but-not-dirty small pages, and another sp for
>> writeable-and-dirty small pages.
>>
>>
> It looks like it can't solve all problems, it fix the access corrupted,
> but will cause D bit losed:
>
> mapping A and mapping B both are writable-and-dirty, when mapping A write
> #PF occurs, the mapping is writable, then we can't set B's D bit anymore.
>

If B is writeable-and-dirty, then it's D bit is already set, and we
don't need to do anything.

If B is writeable-and-clean, then we'll have an spte pointing to a
read-only sp, so we'll get a write fault on access and an opportunity to
set the D bit.

> Anyway, i think we should re-intall the mapping when the state is changed. :-(
>

When the gpte is changed from read-only to writeable or from clean to
dirty, we need to update the spte, yes. But that's true for other sptes
as well, not just large gptes.

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