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

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Date:   Thu, 3 May 2018 14:06:51 +0200
From:   Pierre Morel <pmorel@...ux.vnet.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 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

But since they are not implemented today, I can keep the old name.



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