Re: [PATCH 3/4] xen/arm, libxl: Revert XEN_DOMCTL_shadow_op; use p2m mempool hypercalls

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 16 Nov 2022, Andrew Cooper wrote:
> On 16/11/2022 08:30, Jan Beulich wrote:
> > On 16.11.2022 03:00, Stefano Stabellini wrote:
> >> On Wed, 16 Nov 2022, Andrew Cooper wrote:
> >>> On 16/11/2022 01:37, Stefano Stabellini wrote:
> >>>> On Wed, 26 Oct 2022, Andrew Cooper wrote:
> >>>>> This reverts most of commit cf2a68d2ffbc3ce95e01449d46180bddb10d24a0, 
> >>>>> and bits
> >>>>> of cbea5a1149ca7fd4b7cdbfa3ec2e4f109b601ff7.
> >>>>>
> >>>>> First of all, with ARM borrowing x86's implementation, the logic to set 
> >>>>> the
> >>>>> pool size should have been common, not duplicated.  Introduce
> >>>>> libxl__domain_set_p2m_pool_size() as a shared implementation, and use 
> >>>>> it from
> >>>>> the ARM and x86 paths.  It is left as an exercise to the reader to 
> >>>>> judge how
> >>>>> libxl/xl can reasonably function without the ability to query the pool 
> >>>>> size...
> >>>>>
> >>>>> Remove ARM's p2m_domctl() infrastructure now the functioanlity has been
> >>>>> replaced with a working and unit tested interface.
> >>>>>
> >>>>> This is part of XSA-409 / CVE-2022-33747.
> >>>> Genuine question: I can see this patch removes the implementation of
> >>>> XEN_DOMCTL_SHADOW_OP_SET_ALLOCATION on ARM. It also switches libxl (both
> >>>> ARM and x86) to the new hypercall.
> >>>>
> >>>> Why keep the old hypercall (XEN_DOMCTL_shadow_op and
> >>>> XEN_DOMCTL_SHADOW_OP_SET_ALLOCATION) implementation on x86 (not on ARM)?
> >>>>
> >>>> Is that because it was only recently implemented? And not actually
> >>>> present in any past Xen release?
> >>>>
> >>>> If so, please add a note about this in the commit message. Also, if that
> >>>> is the case, I think this patch series should go in 4.17. If it is too
> >>>> late to get it in before the release, then we should backport it to 4.17
> >>>> as soon as possible. That's because ideally we want to keep the
> >>>> hypercall interface changes down to a minimum.
> >>> On ARM, the hypercall has existed for a little over 4 weeks, and isn't
> >>> in any released version of Xen (yet).
> >>>
> >>> On x86, the hypercall has existed for more than a decade, and has known
> >>> out-of-tree users.  It needs to be deprecated properly, which in this
> >>> case means "phased out in the 4.18 cycle once known callers have been
> >>> adapted to the new hypercall".
> >> Understoon. Then I am in favor of getting all 4 patches in 4.17, either
> >> before the release or via backports.
> > Removing something from the domctl interface generally requires bumping
> > the interface version, so some extra care may need applying if such an
> > interface change was to be backported to any stable branch.
> 
> To be clear, I have no plans to remove the x86 "older" interface in this
> patch series.  It will definitely break out of tree users.
> 
> In the 4.18 timeframe, we can see about retiring the older hypercalls,
> but as a non-backportable change.

For ARM, given that XEN_DOMCTL_shadow_op has not been enabled for long,
maybe we can get away without bumping the interface version?