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Re: [qemu-s390x] [PATCH 00/15] s390: vfio-ccw dasd ipl support

From: Jason J. Herne
Subject: Re: [qemu-s390x] [PATCH 00/15] s390: vfio-ccw dasd ipl support
Date: Tue, 8 Jan 2019 11:37:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 12/12/18 9:34 AM, Cornelia Huck wrote:

NOTE: It has been a while, but I've finally chased down my infamous "reset bug".
On subsystem reset (I see this right after host ipl) we sometimes end up getting
an unexpected unit check status from a dasd device. This causes the first start
subchannel instruction to fail due to the pending unit check status. My solution
to this problem, as advised by the kernel folks, is to simply retry my ssch
instructions before declaring failure when unexpected unit checks happen. In the
event of a persistent error, after two retries we'll give up and print some
useful error info for the user.

So, is that a status we only see because the vfio-ccw driver keeps the
subchannel enabled (as by the other recent thread)?

Is there any value in distinguishing different unit checks, or is retry
the best strategy in any case?

The status presents on device reset. So when the host kernel IPLs this status will be present. The very first attempt to use the device (SSCH, other instructions perhaps?) will cause this status to be presented. Sometimes the host kernel must "get there first" and clear the status. And other times the guest (by way of Qemu bios) gets there first.

The kernel handles unexpected unit checks by simply retrying a low number of times before giving up. Given that bios code is a constant frequency code path, and the kernel has already set this precedent, I feel safe with this decision and don't see a ton of value in doing much more. If we find a case that requires more handling we can take a look at it.

-- Jason J. Herne (address@hidden)

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