On Fri, 24 Sep 2021, 11:21 Jan Beulich, <jbeulich@suse.com> wrote:

> On 24.09.2021 04:30, Julien Grall wrote:
> > Hi Roger,
> >
> > On Thu, 23 Sep 2021, 16:20 Roger Pau Monné, <roger.pau@citrix.com>
> 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@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.
> >>>
> >>> 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.
>
> To be honest using this in such a way would feel like an abuse / layering
> violation to me.
>

I can see how it can be seen like this. Do you have a better suggestion to
be able to map mapping without the foreign mapping interface and the grant
table?

Jan
>
>