Re: pv_ops & gntdev?

[Jeremy Fitzhardinge <jeremy@goop.org>]

Gerd Hoffmann wrote:
> 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.
>   

Would the unmap operation on the vma be enough to do that?   Hm, I guess 
vm_operations_struct needs something that gets called before the zap 
rather than after (->close).

> 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.
>   

Yes, the unpin needs to make sure it clears out all the grant refs, 
because you just can't unpin otherwise.

The alternative is to hook into the fault handler so that a 
fault-n-emulate operation on the pagetable which fails because of the 
grant reference can be fixed up at that point.  But that might be even 
more intrusive.

    J