Re: [PATCH v2 0/6] gnttab: add per-domain controls

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Sep 22, 2021 at 01:57:02PM +0500, Julien Grall wrote:
> 
> 
> On 22/09/2021 13:21, Roger Pau Monne wrote:
> > Hello,
> 
> Hi Roger,
> 
> > First patch on the series is a trivial change to xenconsoled in order to
> > use xenforeignmemory stable library in order to map the shared console
> > ring instead of the unstable libxc interface. It's reviewed and ready to
> > go in.
> > 
> > Patches 2 and 3 allow setting the host wide command line `gnttab` option
> > on a per domain basis. That means selecting the max allowed grant table
> > version and whether transitive grants are allowed.
> > 
> > The last 3 patches attempt to implement support for creating guests
> > without a grant table. This requires some changes to xenstored in order
> > to partially support guests without a valid ring interface, as the lack
> > of grant table will prevent C xenstored from mapping the shared ring.
> > Note this is not an issue for Ocaml xenstored, as it still uses the
> > foreign memory interface to map the shared ring, and thus won't notice
> > the lack of grant table support on the domain.
> 
> I find a bit odd that the Xenstore support is conditional to whether grant
> table is available. Are you expecting domains with no grant table to have no
> PV drivers (including PV shutdown)?

I don't really expect much, as having guests without grant table is a
developer option right now, if someone wants to make use of them for
any reason it would need some thought.

The other option would be my first proposal to restore foreign mapping
of the xenstore ring on that case:

https://lore.kernel.org/xen-devel/20210917154625.89315-6-roger.pau@citrix.com/

But it's also arguable that a guest not having a grant table should
also likely prevent foreign mapping attempts. Plus such foreign
mapping won't work from stubdomains.

I'm fine with dropping those patches if they turn out to be too
controversial, I think it's an interesting option to be able to
disable the grant table, but I don't have a full picture of how that
could be used in practice. Maybe others have and would be willing to
pick this up.

The xenstored patch is mostly so that I can boot guests without a
grant table using xl and test it's disabled using XTF.

Regards, Roger.