linux-kernel - Re: [PATCH v2 01/10] vfio: ccw: Moving state change out of IRQ context

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Message-Id: <5330d3e5-4098-e936-de57-5f4cb2d8f564@linux.ibm.com>
Date:   Tue, 5 Jun 2018 15:34:52 +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 v2 01/10] vfio: ccw: Moving state change out of IRQ
 context

On 04/06/2018 15:52, Cornelia Huck wrote:
> On Fri, 25 May 2018 12:21:09 +0200
> Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
>
>> Let's move the state change from the IRQ routine to the
>> workqueue callback.
>>
>> Signed-off-by: Pierre Morel <pmorel@...ux.vnet.ibm.com>
>> ---
>>   drivers/s390/cio/vfio_ccw_drv.c | 20 +++++++-------------
>>   drivers/s390/cio/vfio_ccw_fsm.c | 14 ++++++++------
>>   2 files changed, 15 insertions(+), 19 deletions(-)
> This causes a change in behaviour for devices in the notoper state.
>
> Now:
> - vfio_ccw_sch_irq is called

This should not be done if the subchannel is not operational.

> - via the state machine, disabling the subchannel is (re-)triggered

I removed the fsm_disabled_irq() callback from VFIO_CCW_STATE_NOT_OPER
because the subchannel is not even initialized at that moment.
We have no reference to the subchannel.

In the previous driver NOT_OPER and STANDBY were quite the same.
Now NOT_OPER means "we can not operate on this sub channel"
because we do not have it in a correct state (no ISC, no mediated device,
the probe is not finiched)

Now STANDBY means we have the device ready but is disabled.
In this case the software infrastructure is ready and if an interrupt comes
(what should not happen) we will disable the subchannel again.

>
> With your patch:
> - the work function is queued in any case; eventually, it will change
>    the device's state to idle (unless we don't have an mdev at that
>    point in time)
> - completion is signaled
>
> I'm not sure that's what we want.
>

Yes it is queued in any case but the IRQ is really treated only if the
subchannel is in the right state (STANDBY, BUSY, IDLE and QUIESCING).

In the NOT_OPER state we do not have the mdev not the driver initialized.

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