Re: [kvm-devel] [PATCH] export notifier #1

[Gerd Hoffmann <kraxel@redhat.com>]

Andrea Arcangeli wrote:
> Like Avi said, Xen is dealing with the linux pte only, so there's no
> racy smp page fault to serialize against. Perhaps we can add another
> notifier for Xen though.
> 
> But I think it's still not enough for Xen to have a method called
> before the ptep_clear_flush: rmap.c would get confused in
> page_mkclean_one for example.

The current code sets a bunch of vma flags (VM_RESERVED, VM_DONTCOPY,
VM_FOREIGN) so the VM doesn't try to handle those special mapping.  IIRC
one of them was needed to not make rmap unhappy.

> Nevertheless if you've any idea on how to use the notifiers for Xen
> I'd be glad to help. Perhaps one workable way to change my patch to
> work for you could be to pass the retval of ptep_clear_flush to the
> notifiers themself. something like:
> 
> #define ptep_clear_flush(__vma, __address, __ptep)			\
> ({									\
> 	pte_t __pte;							\
> 	__pte = ptep_get_and_clear((__vma)->vm_mm, __address, __ptep);	\
> 	flush_tlb_page(__vma, __address);				\
> 	__pte = mmu_notifier(invalidate_page, (__vma)->vm_mm, __address, __pte, __ptep);	\
> 	__pte;								\
> })

Would not work.  Need to pass a pointer to the pte so the xen hypervisor
can do unmap (aka pte_clear) and grant release as atomic operation.
Thus passing the value of the pte entry isn't good enougth.

Another maybe workable approach for Xen is to go through pv_ops
(although pte_clear doesn't go through pv_ops right now, so this would
be an additional hook too ...).

cheers,
  Gerd