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

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

Hi Roger,

On Thu, 23 Sep 2021, 16:20 Roger Pau Monné, <roger.pau@xxxxxxxxxx> wrote:

On Thu, Sep 23, 2021 at 01:47:37PM +0500, Julien Grall wrote:
> Hi Roger,
>
> On 22/09/2021 14:39, Roger Pau Monné wrote:
> > 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@xxxxxxxxxx/
> >
> > 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.
>
> There is another option: extend the acquire hypercall to allow xenstored
> domain to map the xenstore interface. This would require more work, but at
> least it would avoid the interesting dependency on the grant table.

Xen isn't aware of the shared xenstore ring page currently, so that
would mean introducing more knowledge to the hypervisor that what's
strictly required IMO, as Xen has no business in knowing such details.

Well Xen already knows the page for HVM/PVH because the guest retrieve it through an HMV param.

We only miss (?) the PV part.

The grant table slot used by the xenstore shared page is just an
agreement at the toolstack level, but not known to the hypervisor so
far.

Right, we need to find a different way to provide/map the shared page if the grant table is not present.

To me the acquire hypercall is the best way to resolve it as Xen knows whether the domain will run Xenstored (at least we used to have a flag) and we can do the permission control easily.

Do you have another alternative?

> >
> > 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.
>
> I think the current approach is probably OK as a developper option. However,
> we should at least document in the option that disabling the grant-table
> will also disable Xenstore (anything else?) support when using C Xenstored.
>
> It might also be worth to clearly state in the doc that this is only
> intended for developer use and not supported.

Sure, adding it to xl.cfg man page is likely the best place. Will do
when updating the patches.

Thanks, Roger.