Re: [PATCH RFC] xen/grant_table: deprecate GNTTABOP_unmap_and_replace

[David Vrabel <david.vrabel@citrix.com>]

On 21/07/13 18:34, Stefano Stabellini wrote:
> GNTTABOP_unmap_and_replace has two issues:
> - it unconditionally replaces the mapping passed in new_addr with 0;
> - it doesn't support GNTMAP_contains_pte mappings on x86, returning a
> general error instead of some forms of ENOSYS.

I don't think you can change the behavior of the existing sub-op in this
way.  XenServer's current netback driver relies on the current
behaviour, for example.

I think this will also mess up the ref counting on the frame that is at
the new_addr virtual address.  It will be mapped twice but with no
additional ref count.

I think the you can fix the problem in Linux without any hypervisor side
changes, but adding a new sub-op for GNTTABOP_unmap_and_duplicate (unmap
and replace with mapping with a duplicate of an existing mapping) will
allow for batching.

David