Re: [Xen-devel] [PATCH v4 2/2] docs/designs: Add a design document for migration of xenstore data

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Jan 29, 2020 at 02:47:02PM +0000, Paul Durrant wrote:

<snip>

> +**node data**
> +
> +
> +`<path>|<value>|<perm-as-string>|`
> +
> +
> +`<path>` is considered relative to the domain path `/local/domain/$domid`
> +and hence must not begin with `/`.

How backend settings are going to be serialized? For example vif backend
has a bunch of feature-* entries, which should not change under the
guest feet during non-cooperative migration.

> +`<path>` and `<value>` should be suitable to formulate a `WRITE` operation
> +to the receiving xenstore and `<perm-as-string>` should be similarly suitable
> +to formulate a subsequent `SET_PERMS` operation.
> +
> +**watch data**
> +
> +
> +`<path>|<token>|`
> +
> +`<path>` again is considered relative and, together with `<token>`, should
> +be suitable to formulate an `ADD_DOMAIN_WATCHES` operation (see below).
> +
> +
> +### Protocol Extension
> +
> +The `WATCH` operation does not allow specification of a `<domid>`; it is
> +assumed that the watch pertains to the domain that owns the shared ring
> +over which the operation is passed. Hence, for the tool-stack to be able
> +to register a watch on behalf of a domain a new operation is needed:
> +
> +```
> +ADD_DOMAIN_WATCHES      <domid>|<watch>|+
> +
> +Adds watches on behalf of the specified domain.
> +
> +<watch> is a NUL separated tuple of <path>|<token>. The semantics of this
> +operation are identical to the domain issuing WATCH <path>|<token>| for
> +each <watch>.
> +```

Normal WATCH operation triggers an event immediately. Is it intended in
this case too? On the other hand, guest should cope with spurious watch
events, so probably not an issue.

> +The watch information for a domain also needs to be extracted from the
> +sending xenstored so the following operation is also needed:
> +
> +```
> +GET_DOMAIN_WATCHES      <domid>|<index>   <gencnt>|<watch>|* 
> +
> +Gets the list of watches that are currently registered for the domain.
> +
> +<watch> is a NUL separated tuple of <path>|<token>. The sub-list returned
> +will start at <index> into the the overall list of watches and may be
> +truncated such that the returned data fits within XENSTORE_PAYLOAD_MAX.
> +If <index> is beyond the end of the overall list then the returned sub-
> +list will be empty. If the value of <gencnt> changes then it indicates
> +that the overall watch list has changed and thus it may be necessary
> +to re-issue the operation for previous values of <index>.
> +```

In what units <index> is expressed? bytes? entries?
Can the response be truncated at arbitrary place, or only between
records?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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