Re: [PATCH v2 2/6] gnttab: allow setting max version per-domain

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Oct 15, 2021 at 11:48:33AM +0200, Jan Beulich wrote:
> On 15.10.2021 11:39, Jan Beulich wrote:
> > On 22.09.2021 10:21, Roger Pau Monne wrote:
> >> --- a/xen/include/public/domctl.h
> >> +++ b/xen/include/public/domctl.h
> >> @@ -87,14 +87,22 @@ struct xen_domctl_createdomain {
> >>      /*
> >>       * Various domain limits, which impact the quantity of resources
> >>       * (global mapping space, xenheap, etc) a guest may consume.  For
> >> -     * max_grant_frames and max_maptrack_frames, < 0 means "use the
> >> -     * default maximum value in the hypervisor".
> >> +     * max_grant_frames, max_maptrack_frames and max_gnttab_version < 0
> >> +     * means "use the default maximum value in the hypervisor".
> >>       */
> >>      uint32_t max_vcpus;
> >>      uint32_t max_evtchn_port;
> >>      int32_t max_grant_frames;
> >>      int32_t max_maptrack_frames;
> >>  
> >> +/* Grant version, use low 4 bits. */
> >> +#define XEN_DOMCTL_GRANT_version_mask    0xf
> >> +#define XEN_DOMCTL_GRANT_version_default 0xf
> >> +
> >> +#define XEN_DOMCTLGRANT_MAX XEN_DOMCTL_GRANT_version_mask
> >> +
> >> +    uint32_t grant_opts;
> > 
> > As it now seems unlikely that this will make 4.16, please don't forget
> > to bump the interface version for 4.17. With that and preferably with
> > the nit above addressed, hypervisor parts:
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Actually no, I'm afraid there is an issue with migration: If the tool
> stack remembers the "use default" setting and hands this to the new
> host, that host's default may be different from the source host's. It
> is the effective max-version that needs passing on in this case, which
> in turn requires a means to obtain the value.

Hm, my thinking was that the admin (or a higer level orchestration
tool) would already have performed the necessary adjustments to assert
the environments are compatible.

This problem already exist today where you can migrate a VM from a
host having the max default grant version to one having
gnttab=max-ver:1 without complains.

Note that adding such a check would then effectively prevent us from
lowering the default max grant version, as any incoming migration from
a previous hypervisor using the default parameters would be rejected.

Thanks, Roger.