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

[andrew.cooper3@xxxxxxxxxx]

On 03/05/2023 4:22 pm, Juergen Gross wrote:
> On 03.05.23 17:15, Julien Grall wrote:
>> 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;
>
> I think there are barriers missing (especially in order to work on Arm)?

Yes there are.  I think x86 skates by on side effects of hypercalls.

>
> And I think you will break dom0 with calling domain_conn_reset(), as the
> kernel might already have written data into the xenbus page. So you might
> want to make the call depend on !is_master_domain.

And this is why I am very deliberately not doing anything until the
documentation is matches reality, and is safe to use.

For starters, shuffling this doesn't make any difference for a domU
which hasn't been taught about this optional extension.  Ignoring such
cases is not an acceptable fix.

~Andrew