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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.10.2021 10:04, Roger Pau Monné wrote:
> 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.

I don't think we can rely on this in the hypervisor. We may take this
as a prereq for proper working, but I think we ought to detect
violations and report errors in such a case.

> 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.

Are you sure about "without complaints"? What would a guest do if it
expects to be able to use v2, since it was able to on the prior host?

> 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.

Right, guests would need booting anew on a such restricted host.

Jan