Re: [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Aug 13, 2013 at 12:17:18PM +0100, Stefano Stabellini wrote:
> On Fri, 9 Aug 2013, Konrad Rzeszutek Wilk wrote:
> > On Sun, Aug 04, 2013 at 03:39:41PM +0100, Stefano Stabellini wrote:
> > > GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
> > > mapping instead of reinstating the original mapping.
> > > Doing so separately would be racy.
> > > 
> > > To unmap a grant and reinstate the original mapping atomically we use
> > > GNTTABOP_unmap_and_replace.
> > > GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
> > > don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
> > > passed in new_addr so we have to reinstate it, however that is a
> > > per-cpu mapping only used for balloon scratch pages, so we can be sure that
> > > it's not going to be accessed while the mapping is not valid.
> > 
> > This looks to be depend on a new structure, which is not in Linux kernel?
> > Are you missing a dependency patch?
> 
> Nope, GNTTABOP_unmap_and_replace and struct gnttab_unmap_and_replace are
> already present in include/xen/interface/grant_table.h.
> 
> 
> > Shouldn't we use some logic to figure out which hypercall to use if the
> > hypervisor does not support it?
> 
> GNTTABOP_unmap_and_replace is not a new hypercall, it has been supported
> by Xen for a very long time.
> 
> In a previous iteration of this patch series, I did introduce a new
> hypercall, but then I dropped it because I figured out that I could
> achieve the same thing with the existing hypercall.

OK, Please tack on:

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

P.S.
If you could stick it on devel/for-linus-3.12 that would be super. Thanks!