[Xen-devel] Re: pv_ops & gntdev?

[Gerd Hoffmann <kraxel@xxxxxxxxxx>]

Jeremy Fitzhardinge wrote:
> Gerd Hoffmann wrote:
>> Hmm.  One more Q:  How is pinning used?  Is a process pinned all the
>> time?  Or can it happen that the pages are unpinned in case it does hang
>> around idle for a while?  Is there some way to prevent a process from
>> being unpinned?
>>   
> 
> Pinning in the sense of Xen pagetables; a pagetable is pinned for the
> whole time the process exists - it gets pinned at fork/exec time, and
> unpin on exit.  (It has no effect on the residency of the usermode
> pages, though granted pages mapped into usermode would not be swappable
> at all.)
> 
>> I somehow feel like checking out mmu notifiers first ...
>>   
> Why's that?  What would you use them for?

Unmapping the grants when the kernel is about to zap the page range.
I think it could be easier because I don't have to link the low-level
paravirt code with the gntdev driver then.  Also I can easily batch
unmaps.  And there is a performance hit for the extra checks only for
processes which actually use grants.

The (unclean) exit case might be tricky though.  Could be the kernel
tries to unpin before zapping all mappings, so we don't trap into xen
all the time.

cheers,
  Gerd


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