Re: dom0less vs xenstored setup race Was: xen | Failed pipeline for staging | 6a47ba2f

[Julien Grall <julien@xxxxxxx>]

Hi,

On 03/05/2023 15:38, andrew.cooper3@xxxxxxxxxx wrote:
> Hello,
>
> After what seems like an unreasonable amount of debugging, we've tracked
> down exactly what is going wrong here.
>
> https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/4219721944
>
> Of note is the smoke.serial log around:
>
> io: IN 0xffff90fec250 d0 20230503 14:20:42 INTRODUCE (1 233473 1 )
> obj: CREATE connection 0xffff90fff1f0
> *** d1 CONN RESET req_cons 00000000, req_prod 0000003a rsp_cons
> 00000000, rsp_prod 00000000
> io: OUT 0xffff9105cef0 d0 20230503 14:20:42 WATCH_EVENT
> (@introduceDomain domlist )
>
> XS_INTRODUCE (in C xenstored at least, not checked O yet) always
> clobbers the ring pointers.  The added pressure on dom0 that the
> xensconsoled adds with it's 4M hypercall bounce buffer occasionally
> defers xenstored long enough that the XS_INTRODUCE clobbers the first
> message that dom1 wrote into the ring.
>
> The other behaviour seen was xenstored observing a header looking like this:
>
> *** d1 HDR { ty 0x746e6f63, rqid 0x2f6c6f72, txid 0x74616c70, len
> 0x6d726f66 }
>
> which was rejected as being too long.  That's "control/platform" in
> ASCII, so the XS_INTRODUCE intersected dom1 between writing the header
> and writing the payload.
>
>
> Anyway, it is buggy for XS_INTRODUCE to be called on a live an
> unsuspecting connection.  It is ultimately init-dom0less's fault for
> telling dom1 it's good to go before having waited for XS_INTRODUCE to
> complete.
So the problem is xenstored will set interface->connection to 
XENSTORE_CONNECTED before finalizing the connection. Caqn you try the 
following, for now, very hackish patch:
diff --git a/tools/xenstore/xenstored_domain.c 
b/tools/xenstore/xenstored_domain.c
index f62be2245c42..bbf85bbbea3b 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/tools/xenstore/xenstored_domain.c
@@ -688,6 +688,7 @@ static struct domain *introduce_domain(const void *ctx,
                talloc_steal(domain->conn, domain);

                if (!restore) {
+                       domain_conn_reset(domain);
                        /* Notify the domain that xenstore is available */
                        interface->connection = XENSTORE_CONNECTED;
                        xenevtchn_notify(xce_handle, domain->port);
@@ -730,8 +731,6 @@ int do_introduce(const void *ctx, struct connection 
*conn,
        if (!domain)
                return errno;

-       domain_conn_reset(domain);
-
        send_ack(conn, XS_INTRODUCE);

        return 0;

Cheers,

--
Julien Grall