Re: [PATCH v2 2/4] tools/xenstore: add documentation for new set/get-feature commands

[Julien Grall <julien@xxxxxxx>]

Hi Juergen,

On 27/05/2022 08:24, Juergen Gross wrote:
> Add documentation for two new Xenstore wire commands SET_FEATURE and
> GET_FEATURE used to set or query the Xenstore features visible in the
> ring page of a given domain.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> Do we need support in the migration protocol for the features?
I would say yes. You want to make sure that the client can be migrated 
without loosing features between two xenstored.
> V2:
> - remove feature bit (Julien Grall)
> - GET_FEATURE without domid will return Xenstore supported features
>    (triggered by Julien Grall)
> ---
>   docs/misc/xenstore.txt | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
>
> diff --git a/docs/misc/xenstore.txt b/docs/misc/xenstore.txt
> index a3d3da0a5b..00f6969202 100644
> --- a/docs/misc/xenstore.txt
> +++ b/docs/misc/xenstore.txt
> @@ -331,6 +331,20 @@ SET_TARGET         <domid>|<tdomid>|
>   
>   	xenstored prevents the use of SET_TARGET other than by dom0.
>   
> +GET_FEATURE		[<domid>|]		<value>|
> +SET_FEATURE            <domid>|<value>|
> +       Returns or sets the contents of the "feature" field located at
> +       offset 2064 of the Xenstore ring page of the domain specified by
> +       <domid>. <value> is a decimal number being a logical or of the
In the context of migration, I am stilll a bit concerned that the 
features are stored in the ring because the guest could overwrite it.
I would expect the migration code to check that the GET_FEATURE <domid> 
is a subset of GET_FEATURE on the targeted Xenstored. So it can easily 
prevent a guest to migrate.
So I think this should be a shadow copy that will be returned instead of 
the contents of the "feature" field.
> +       feature bits as defined in docs/misc/xenstore-ring.txt. Trying
> +       to set a bit for a feature not being supported by the running
> +       Xenstore will be denied. Providing no <domid> with the
> +       GET_FEATURE command will return the features which are supported
> +       by Xenstore.
Do we want to allow modifying the features when the guest is running?

> +
> +       xenstored prevents the use of GET_FEATURE and SET_FEATURE other
> +       than by dom0.
Cheers,

--
Julien Grall