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

[andrew.cooper3@xxxxxxxxxx]

On 03/05/2023 10:53 pm, Stefano Stabellini wrote:
> On Wed, 3 May 2023, Julien Grall wrote:
>> 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);
> Following Jurgen's suggestion, I made this slightly modified version of
> the patch. With it, the problem is solved:
>
> https://gitlab.com/xen-project/people/sstabellini/xen/-/pipelines/856450703

This fails to solve 3(?) of the 4(?) bugs pointed out between this email
thread and on IRC.

Stop with the bull-in-a-china-shop approach.  There is no acceptable fix
to this mess which starts with anything other than corrections to the
documentation, and a plan for how to make startup work robustly given
all the bugs introduced previously by failing to do it properly the
first time around.

~Andrew