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Re: [qemu-s390x] [0/3] vfio-ccw: support hsch/csch (kernel part)

From: Jason J. Herne
Subject: Re: [qemu-s390x] [0/3] vfio-ccw: support hsch/csch (kernel part)
Date: Wed, 5 Dec 2018 13:08:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1


Sorry if this does not thread nicely. I never received the original (Thunderbird corruption/hangs) so I had to fake it :).

>> My point is, when the subchannel is disabled, 'firmware' is responsible
>> for suppressing interrupts and error conditions, and also for
>> doing the appropriate recovery procedure, so to say under the hood.
> I don't think there's actually much of a 'recovery' possible at a
> subchannel level (other than 'have you tried turning it off and on
> again?'); the interesting stuff is all at the device-specific level.
>> I think Jason has discovered some problems related to this while doing
>> his DASD IPL with vfio-ccw work, but I don't quite remember any more.
> cc:ing Jason, in case he remembers :)

Here is what Halil was talking about.
I'm seeing a problem during kvm on z development of vfio-ccw (passthrough dasd). After a fresh IPL of the host system, sometimes my first channel program executed on my vfio-ccw device generates a unit check. The sense data given for that unit check indicates that a reset event has occurred. This is apparently normal to see after a device, channel or subsystem reset.

I'm trying to figure out how to deal with this unit check in the vfio-ccw kernel driver. The thinking at the moment is to just retry any i/o operation a limited number of times after any unit check, then give up if the i/o operation still does not succeed. The kernel apparently uses a similar approach for the regular dasd driver.

-- Jason J. Herne (address@hidden)

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