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Violators will be prosecuted for from ; Thu, 12 Dec 2019 17:33:19 -0000 Subject: Re: [kvm-unit-tests PATCH v4 7/9] s390x: css: msch, enable test From: Pierre Morel References: <1576079170-7244-1-git-send-email-pmorel@linux.ibm.com> <1576079170-7244-8-git-send-email-pmorel@linux.ibm.com> <20191212130111.0f75fe7f.cohuck@redhat.com> <83d45c31-30c3-36e1-1d68-51b88448f4af@linux.ibm.com> <20191212151002.1c7ca4eb.cohuck@redhat.com> <20191212153303.6444697e.cohuck@redhat.com> Date: Thu, 12 Dec 2019 18:33:15 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Language: en-US Content-Transfer-Encoding: 8bit Message-Id: Sender: linux-s390-owner@vger.kernel.org List-ID: To: Cornelia Huck Cc: kvm@vger.kernel.org, linux-s390@vger.kernel.org, frankja@linux.ibm.com, david@redhat.com, thuth@redhat.com On 2019-12-12 17:05, Pierre Morel wrote: > > > On 2019-12-12 15:33, Cornelia Huck wrote: >> On Thu, 12 Dec 2019 15:21:21 +0100 >> Pierre Morel wrote: >> >>> On 2019-12-12 15:10, Cornelia Huck wrote: >>>> On Thu, 12 Dec 2019 15:01:07 +0100 >>>> Pierre Morel wrote: >>>>> On 2019-12-12 13:01, Cornelia Huck wrote: >>>>>> On Wed, 11 Dec 2019 16:46:08 +0100 >>>>>> Pierre Morel wrote: >>>>>>> A second step when testing the channel subsystem is to prepare a >>>>>>> channel >>>>>>> for use. >>>>>>> This includes: >>>>>>> - Get the current SubCHannel Information Block (SCHIB) using STSCH >>>>>>> - Update it in memory to set the ENABLE bit >>>>>>> - Tell the CSS that the SCHIB has been modified using MSCH >>>>>>> - Get the SCHIB from the CSS again to verify that the subchannel is >>>>>>>      enabled. >>>>>>> >>>>>>> This tests the success of the MSCH instruction by enabling a >>>>>>> channel. >>>>>>> >>>>>>> Signed-off-by: Pierre Morel >>>>>>> --- >>>>>>>     s390x/css.c | 65 >>>>>>> +++++++++++++++++++++++++++++++++++++++++++++++++++++ >>>>>>>     1 file changed, 65 insertions(+) >>>>>>> +    /* Read the SCHIB for this subchannel */ >>>>>>> +    cc = stsch(test_device_sid, &schib); >>>>>>> +    if (cc) { >>>>>>> +        report(0, "stsch cc=%d", cc); >>>>>>> +        return; >>>>>>> +    } >>>>>>> + >>>>>>> +    /* Update the SCHIB to enable the channel */ >>>>>>> +    pmcw->flags |= PMCW_ENABLE; >>>>>>> + >>>>>>> +    /* Tell the CSS we want to modify the subchannel */ >>>>>>> +    cc = msch(test_device_sid, &schib); >>>>>>> +    if (cc) { >>>>>>> +        /* >>>>>>> +         * If the subchannel is status pending or >>>>>>> +         * if a function is in progress, >>>>>>> +         * we consider both cases as errors. >>>>>>> +         */ >>>>>>> +        report(0, "msch cc=%d", cc); >>>>>>> +        return; >>>>>>> +    } >>>>>>> + >>>>>>> +    /* >>>>>>> +     * Read the SCHIB again to verify the enablement >>>>>>> +     * insert a little delay and try 5 times. >>>>>>> +     */ >>>>>>> +    do { >>>>>>> +        cc = stsch(test_device_sid, &schib); >>>>>>> +        if (cc) { >>>>>>> +            report(0, "stsch cc=%d", cc); >>>>>>> +            return; >>>>>>> +        } >>>>>>> +        delay(10); >>>>>> >>>>>> That's just a short delay to avoid a busy loop, right? msch should be >>>>>> immediate, >>>>> >>>>> Thought you told to me that it may not be immediate in zVM did I >>>>> misunderstand? >>>> >>>> Maybe I have been confusing... what I'm referring to is this >>>> programming note for msch: >>>> >>>> "It is recommended that the program inspect the >>>> contents of the subchannel by subsequently >>>> issuing STORE SUBCHANNEL when MODIFY >>>> SUBCHANNEL sets condition code 0. Use of >>>> STORE SUBCHANNEL is a method for deter- >>>> mining if the designated subchannel was >>>> changed or not. Failure to inspect the subchan- >>>> nel following the setting of condition code 0 by >>>> MODIFY SUBCHANNEL may result in conditions >>>> that the program does not expect to occur." >>>> >>>> That's exactly what we had to do under z/VM back then: do the msch, >>>> check via stsch, redo the msch if needed, check again via stsch. It >>>> usually worked with the second msch the latest. >>> >>> OK, I understand, then it is a bug in zVM that this test could >>> enlighten. >> >> Probably more a quirk than a bug... the explanation there is not >> explicit about that :) >> >>> >>> I think we should keep it so, it allows to recognize 3 cases (after I >>> change to test ENABLE in the loop as I said I will): >>> - immediate ENABLE >> >> This is the good case. >> >>> - asynchrone ENABLE >> >> This one I would consider an architecture violation. >> >>> - failure to ENABLE >> >> This is the quirk above. >> >> But I'm not quite sure how you would be able to distinguish the last >> two cases? > > This is where the delay can help. > But yes, not sure that we can differentiate this without to know how > long we should delay. > > >> >>>>>> and you probably should not delay on success? >>>>> >>>>> yes, it is not optimized, I can test PMCW_ENABLE in the loop this >>>>> way we >>>>> can see if, in the zVM case we need to do retries or not. >>>>> >>>>>>> +    } while (!(pmcw->flags & PMCW_ENABLE) && count++ < 5); >>>>>> >>>>>> How is this supposed to work? Doesn't the stsch overwrite the control >>>>>> block again, so you need to re-set the enable bit before you retry? >>>>> >>>>> I do not think so, there is no msch() in the loop. >>>>> Do I miss something? >>>> >>>> Well, _I_ missed that the msch() was missing :) You need it (see >>>> above); >>>> just waiting and re-doing the stsch is useless, as msch is a >>>> synchronous instruction which has finished its processing after the cc >>>> has been set. >>> >>> Since kvm-unit-test is a test system, not an OS so I think that here we >>> have one more point to leverage the enable function: >>> - We need to test the enable (what I did (partially)) >> >> Maybe also log if you needed to retry? Not as an error, but as >> additional information? > > Yes. > > Regards, > Pierre > After all, I make it simple by testing if the MSCH works as expected, no retry, no delay. This is just a test. I will add a new patch to add a library function to enable the channel, with retry to serve when we really need to enable the channel, not to test. Regards, Pierre -- Pierre Morel IBM Lab Boeblingen