Re: [Xen-devel] Live migration leaves page tables read-only?

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 9/12/06 9:22 am, "Keir Fraser" <keir@xxxxxxxxxxxxx> wrote:

>> Although your patch works today, it will break when we add PSE (super
>> page) support for PV guests as it will confuse PROT_NONE with PSE.
>> Assuming PROT_NONE only makes sense for L1 entries, we can probably gate
>> the tests on whether the page table page is an L1 or not to fix this.
>> 
>> However, it does point out an issue for other OSes: Taking this patch
>> effectively makes Linux's PROT_NONE (flags 0x80 for a not present PTE)
>> part of the Xen API. We need to find out whether this is compatible with
>> *BSD and Solaris' use of flags for not present ptes.
> 
> If _PAGE_PRESENT is clear then the other N-1 bits can be assumed available
> for things like swapcache info. Making assumptions about not-present PTEs is
> not really tenable.

Speaking more constructively we could have a pte_active_mask communicated
via elfnote or xenbus (or some other way) which the tools would apply to
PTEs to determine if they contain an MFN. Default would be 0x1.

 -- Keir


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