Re: [Xen-devel] [PATCH RFC] xen: prevent PVH Dom0 from having pages with more than one ref

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 06/13/14 1:17 PM >>>
>On 13/06/14 12:05, Jan Beulich wrote:
>>>>> Roger Pau MonnÃ<roger.pau@xxxxxxxxxx> 06/13/14 12:17 PM >>>
>>> AFAICT (given my little knowledge about the domain builder), those pages
>>> have been allocated by alloc_domheap_pages earlier in the code, which
>>> makes them already have 1 ref, and then this loop adds another ref to
>>> them, which makes no sense to me.
>> alloc_domheap_pages() (or really assign_pages()) takes one reference for
>> setting PGC_allocated. This is the reference that gets dropped when freeing
>> the page. The code you modify takes a second reference for the usage type
>> of the page becoming writable, which is a result of it getting a writable 
>> page
>> table entry made for it. That reference gets dropped when the respective
>> page table entry gets cleared.
>
>Ah, so it is a genuine difference between PV and HVM memory management.
>
>Perhaps the conditional should be based on paging_mode_translate() ?

That would make sense, yes.

Jan


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