Write bit in Shadow Page Table Entry

Write bit in Shadow Page Table Entry

[Lynda Yang]

Hi,
As I understand, the KVM code (kvm-86) may clear the write bit when
setting a shadow page table entry so that it can detect when an entry
needs to be marked dirty later. However, it also plays with the write
bit depending on whether the shadow page is allowed to be unsynched or
not. I'm not quite clear on the latter, so if anyone can provide some
insights it would be very much appreciated. Or perhaps even more
helpful...if it is possible to provide a clear picture of how KVM
generally plays with an entry's write bit.

Thank you!,
-Lynda Yang

Re: Write bit in Shadow Page Table Entry

[Avi Kivity]

On 07/23/2009 11:49 PM, Lynda Yang wrote:
> Hi,
> As I understand, the KVM code (kvm-86) may clear the write bit when
> setting a shadow page table entry so that it can detect when an entry
> needs to be marked dirty later. However, it also plays with the write
> bit depending on whether the shadow page is allowed to be unsynched or
> not. I'm not quite clear on the latter, so if anyone can provide some
> insights it would be very much appreciated. Or perhaps even more
> helpful...if it is possible to provide a clear picture of how KVM
> generally plays with an entry's write bit.
>    

Let's see.

1. If the spte was derived from a guest pte, then the writeable bit 
reflects the guest permissions.  If not (nested paging, real mode), the 
writeable bit is 1.
2. When we log dirty pages, writeable bits for the memory we are 
interested in are cleared.
3. If the spte points at a shadow page, the writeable bit is cleared to 
0 so that we are informed of updates to page tables.
4. Under certain conditions[1], we allow a shadowed guest page table to 
be writeable.  This happens on a write fault to a guest page table.

[1] The conditions are: the page is the lowest-level mapping, and there 
are no other uses of the page as a paging element.

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