[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [qemu-s390x] [Qemu-devel] [PATCH v2 0/6] s390x/vfio: VFIO-AP interru

From: Pierre Morel
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v2 0/6] s390x/vfio: VFIO-AP interrupt control interception
Date: Fri, 30 Nov 2018 14:01:42 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 29/11/2018 16:55, Halil Pasic wrote:
On Thu, 22 Nov 2018 17:35:49 +0100
Pierre Morel <address@hidden> wrote:

This series has 3 different type of patches:

The first two:
   s390x/vfio: ap: Finding the AP bridge
   s390x/vfio: ap: Use the APdevice as a child of the APBus

Are dealing with the QEMU Object Model and how we retrieve the
AP devices from instruction interception.
A lifting of the AP bridge, bus and device was necessary to
ease the process and allow future extensions.

The third one is a place holder to ease reviewing:
   s390x/vfio: ap: Linux uapi VFIO place holder

The last three are really dealing with PQAP/AQIC instruction
interception and associate IOCTL calls to the VFIO AP device
   s390x/cpumodel: Set up CPU model for AQIC interception
   s390x/vfio: ap: Definition for AP Adapter type
   s390x/vfio: ap: Implementing AP Queue Interrupt Control

The S390 APQP/AQIC instruction is intercepted by the host
to configure the AP queues interruption redirection.
It retrieves the ISC used by the host and the one used
by the guest and setup the indicator address.

This patch series
- define the AQIC feature in the cpumodel,
- extend the APDevice type for per card and queue interrupt handling,
- intercept the APQP/AQIC instruction, uses the S390 adapter interface
   to setup the adapter
- and use a VFIO ioctl to let the VFIO-AP driver handle the host
   instruction associated with the intercepted guest instruction.

This patch serie can be tested with the Linux/KVM patch series
for the VFIO-AP driver: "s390: vfio: ap: Using GISA for AP Interrupt"

Sorry for raising concern this late, I hope it's better late than

I have strong doubts that handling PQAP/AQCI via userspace is worth
the effort. IMHO we could do what we have to do on AQCI in kernel
iff the ap is done SIE interpreted, the appropriate feature is presented
to the guest, and the queue in question belongs to the given guest. Or
am I wrong?

I do understand that doing it like this *may* end up being beneficial
*if* we decide to do some sort of ap virtualization in QEMU. But I don't
see it coming in the foreseeable future, and for ap virtualization we
would need a solution for making the host do an NQAP and an DQAP on
behalf of the guest/emulator, and not only to do the same for PQAP/QCI.

In my understanding, with this, we would end up with an infrastructure
that only makes sense in a perspective of some 'future features' which
may never come to existence.

What I ask for is, please, let us examine the other option.


As we discussed offline, I already began to implement prototypes for both options.
This is a clear design choice.

If we examine the pro and contra, I can list:

1- Pro kernel implementation of the PQAP/AQIC
-> rapidity of the reaction

Question: is this important?
Answer: NO,
Why: The PQAP/AQIC is rarely used by the AP driver of the guest. exclusively during RESET of the AP queue.
I do not think we need a rapid reaction there.

2- Pro userland implementation of PQAP/AQIC
-> standard implementation, already used by PCI, CCW

Question: is it important?
Answer: YES
Why:  like following the standard
It is easily extend-able to other virtualization implementation like interception based VFIO and emulation

There is no implementation which would be really more complicated as the other, for both we will need to introduce new pro APQN (queue) structures to hold the interrupt information (ISC, NIB), for both we will need to ping the NIB in memory.

So as long as there are no other opinion against the design I presented here I will continue this way while considering the comments I got on this series.


Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany

reply via email to

[Prev in Thread] Current Thread [Next in Thread]