Re: support for sharing huge pages with grant table?

[Ian Campbell <Ian.Campbell@citrix.com>]

On Sun, 2014-11-16 at 23:39 -0500, Tim Wood wrote:
> Hi,
> 
> I am curious if Xen currently supports sharing hugepages between
> domains (specifically ones originally allocated in Dom-0 and shared
> with a guest r/w).  I've seen some references to huge pages in the
> archives of this list, but not in relation to the grant mechanism.

I don't think the grant table has any specific superpage support. It
might be an interesting extension to consider though (for the sorts of
reasons you would like it).

You could grant a superpage using multiple 4K grants to cover whichever
subset of the superpage you need to expose to the other end. Now granted
(no pun intended ;-)) that might suck up 512 grefs in the worst case,
which is a bit mad...

> Also, can someone confirm that "superpages" are another term for "huge
> pages" in Xen?

Yes. Or at least I think so.

> This would be helpful for some work we are doing on high speed
> networking to VMs---DPDK stores packets into huge pages and we'd like
> to get those to VMs as quickly as possible.

This seems like a reasonable usecase to me. Having added this to the
grant table interface I suppose you would also need to consider
extensions to the individual PV I/O protocols (netif.h) to allow them to
signal when a grant was huge. You might have issues with e.g. finding
enough bits to represent the larger sizes...

Ian.