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

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

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

On 24.09.2021 04:30, Julien Grall wrote:
> 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.

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