linux-kernel - Re: [PATCH 04/10] vfio: ccw: replace IO_REQ event with SSCH

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Date:   Wed, 23 May 2018 10:19:18 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:     pasic@...ux.vnet.ibm.com, bjsdjshi@...ux.vnet.ibm.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 04/10] vfio: ccw: replace IO_REQ event with SSCH_REQ event

On 22/05/2018 17:38, Cornelia Huck wrote:
> [still backlog processing...]
>
> On Thu, 3 May 2018 14:06:51 +0200
> Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
>
>> On 30/04/2018 17:30, Cornelia Huck wrote:
>>> On Wed, 25 Apr 2018 15:52:19 +0200
>>> Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
>>>   
>>>> On 25/04/2018 10:41, Cornelia Huck wrote:
>>>>> On Thu, 19 Apr 2018 16:48:07 +0200
>>>>> Pierre Morel<pmorel@...ux.vnet.ibm.com>  wrote:
>>>>>> diff --git a/drivers/s390/cio/vfio_ccw_private.h b/drivers/s390/cio/vfio_ccw_private.h
>>>>>> index 3284e64..93aab87 100644
>>>>>> --- a/drivers/s390/cio/vfio_ccw_private.h
>>>>>> +++ b/drivers/s390/cio/vfio_ccw_private.h
>>>>>> @@ -76,7 +76,7 @@ enum vfio_ccw_state {
>>>>>>      */
>>>>>>     enum vfio_ccw_event {
>>>>>>     	VFIO_CCW_EVENT_NOT_OPER,
>>>>>> -	VFIO_CCW_EVENT_IO_REQ,
>>>>>> +	VFIO_CCW_EVENT_SSCH_REQ,
>>>>>>     	VFIO_CCW_EVENT_INTERRUPT,
>>>>>>     	VFIO_CCW_EVENT_SCH_EVENT,
>>>>>>     	/* last element! */
>>>>> I don't think we should separate the ssch handling. The major
>>>>> difference to halt/clear is that it needs channel program translation.
>>>>> Everything else (issuing the instruction and processing the interrupt)
>>>>> are basically the same. If we just throw everything at the hardware
>>>>> and let the host's channel subsystem figure it out, we already should
>>>>> be fine with regard to most of the races.
>>>> We must test at a moment or another the kind of request we do,
>>>> cancel, halt and clear only need the subchannel id in register 1 and as
>>>> you said are much more direct to implement.
>>>>
>>>> If we do not separate them here, we need a switch in the "do_io_request"
>>>> function.
>>>> Is it what you mean?
>>> Yes. Most of the handling should be the same for any function.
>> I really don't know, the 4 functions are quite different.
>>
>> - SSCH uses an ORB, and has a quite long kernel execution time for VFIO
>> - there is a race between SSCH and the others instructions
>> - XSCH makes subchannel no longer start pending, also reset the busy
>> indications
>> - CSCH cancels both SSCH and HSCH instruction, and perform path management
>> - HSCH has different busy (entry) conditions
> Roughly speaking, we have two categories: An asynchronous function is
> performed (SSCH, HSCH, CSCH) or not (XSCH). So I would split out XSCH
> in any case.
>
> SSCH, HSCH, CSCH all perform path management. I see them as kind of
> escalating (i.e. CSCH 'beats' HSCH which 'beats' SSCH). I think they
> are all similar enough, though, as we can call through to the real
> hardware and have it sorted out there.
>
> Looking through the channel I/O instructions:
> - RSCH should be handled with SSCH (as a special case).
> - MSCH should also be handled in the long run, STSCH as well.
> - SCHM is interesting, as it's not per-subchannel. We have some basic
>    handling of the instruction in QEMU, but it only emulates some ssch
>    counters and completely lacks support for the other fields.
> - IIRC, there's also a CHSC command dealing with channel monitoring. We
>    currently fence off any CHSC that is not needed for Linux to run, but
>    there are some that might be useful for the guest (path handling
>    etc.) Hard to come to a conclusion here without access to the
>    documentation.
> - I don't think we need to care about TSCH (other than keeping the
>    schib up to date, which we also need to do for STSCH).
> - Likewise, TPI should be handled via emulation.
>
> Coming back to the original issue, I think we can easily handle SSCH
> (and RSCH), HSCH and CSCH together (with the actual hardware doing the
> heavy lifting anyway). For other instructions, we need separate
> states/processing.
>

OK, I make the next version with this in mind.

Thanks

Pierre


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany