Re: [PATCH for-4.17 v3 10/15] tools/ocaml/xenstored: keep eventchn FD open across live update

[Christian Lindig <christian.lindig@xxxxxxxxxx>]

> On 8 Nov 2022, at 15:34, Edwin Török <edvin.torok@xxxxxxxxxx> wrote:
> 
> Avoid this by trying to keep the eventchn file descriptor open across
> live updates, using the new xenevtchn_fdopen API, similar to how C
> xenstored works.
> 
> However an old enough oxenstored won't have kept the evtchn fd open and
> won't have saved it in the update stream, so handle that case by
> reopening the evtchn as a fallback.
> 
> A followup commit will avoid rebind the guest remote port (we cannot
> rely on the remote port staying the same, e.g. the windows PV drivers
> change it)
> 
> Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
> ---
> Reason for inclusion in 4.17:
> - fixes live update in oxenstored, making future security updates easier
>  if the base code already has this patch
> 
> Changes since v2:
> - new in v3
> ---
> tools/ocaml/xenstored/domains.ml   |  1 +
> tools/ocaml/xenstored/event.ml     |  7 ++-
> tools/ocaml/xenstored/xenstored.ml | 71 +++++++++++++++++++++---------
> 3 files changed, 56 insertions(+), 23 deletions(-)

Acked-by: Christian Lindig <christian.lindig@xxxxxxxxxx>


> +    let require_doms () =
> +      match !doms with
> +      | None ->
> +        let missing_eventchnfd = !eventchnfd = None in
> +        if missing_eventchnfd then
> +          warn "No event channel file descriptor available in dump!";
> +        let eventchn = Event.init ?fd:!eventchnfd () in
> +        let domains = createdoms eventchn in
> +        if missing_eventchnfd then
> +          Event.bind_dom_exc_virq eventchn;
> +        doms := Some domains;
> +        domains
> +      | Some d -> d

I like to use the “when” clause in these situations in order to keep the match 
cases flat even at the cost of a slight code duplication. But it’s a matter of 
taste.

| None when !eventchnfd = None ->

Or 

match !doms, !eventchnfd with 
| None, None ->
|