Re: [PATCH v5 7/9] s390x/pci: enable adapter event notification for interpreted devices

[Pierre Morel <pmorel@linux.ibm.com>]

On 5/2/22 21:57, Matthew Rosato wrote:
> On 5/2/22 7:30 AM, Pierre Morel wrote:
>>
>>
>> On 5/2/22 11:19, Niklas Schnelle wrote:
>>> On Mon, 2022-05-02 at 09:48 +0200, Pierre Morel wrote:
>>>>
>>>> On 4/22/22 14:10, Matthew Rosato wrote:
>>>>> On 4/22/22 5:39 AM, Pierre Morel wrote:
>>>>>>
>>>>>> On 4/4/22 20:17, Matthew Rosato wrote:
>>>>>>> Use the associated kvm ioctl operation to enable adapter event
>>>>>>> notification
>>>>>>> and forwarding for devices when requested.  This feature will be 
>>>>>>> set up
>>>>>>> with or without firmware assist based upon the 'forwarding_assist'
>>>>>>> setting.
>>>>>>>
>>>>>>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>>>>>>> ---
>>>>>>>    hw/s390x/s390-pci-bus.c         | 20 ++++++++++++++---
>>>>>>>    hw/s390x/s390-pci-inst.c        | 40 
>>>>>>> +++++++++++++++++++++++++++++++--
>>>>>>>    hw/s390x/s390-pci-kvm.c         | 30 +++++++++++++++++++++++++
>>>>>>>    include/hw/s390x/s390-pci-bus.h |  1 +
>>>>>>>    include/hw/s390x/s390-pci-kvm.h | 14 ++++++++++++
>>>>>>>    5 files changed, 100 insertions(+), 5 deletions(-)
>>>>>>>
>>>>>>> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
>>>>>>> index 9c02d31250..47918d2ce9 100644
>>>>>>> --- a/hw/s390x/s390-pci-bus.c
>>>>>>> +++ b/hw/s390x/s390-pci-bus.c
>>>>>>> @@ -190,7 +190,10 @@ void s390_pci_sclp_deconfigure(SCCB *sccb)
>>>>>>>            rc = SCLP_RC_NO_ACTION_REQUIRED;
>>>>>>>            break;
>>>>>>>        default:
>>>>>>> -        if (pbdev->summary_ind) {
>>>>>>> +        if (pbdev->interp && (pbdev->fh & FH_MASK_ENABLE)) {
>>>>>>> +            /* Interpreted devices were using interrupt 
>>>>>>> forwarding */
>>>>>>> +            s390_pci_kvm_aif_disable(pbdev);
>>>>>>
>>>>>> Same remark as for the kernel part.
>>>>>> The VFIO device is already initialized and the action is on this
>>>>>> device, Shouldn't we use the VFIO device interface instead of the KVM
>>>>>> interface?
>>>>>>
>>>>>
>>>>> I don't necessarily disagree, but in v3 of the kernel series I was 
>>>>> told
>>>>> not to use VFIO ioctls to accomplish tasks that are unique to KVM 
>>>>> (e.g.
>>>>> AEN interpretation) and to instead use a KVM ioctl.
>>>>>
>>>>> VFIO_DEVICE_SET_IRQS won't work as-is for reasons described in the
>>>>> kernel series (e.g. we don't see any of the config space notifiers
>>>>> because of instruction interpretation) -- as far as I can figure we
>>>>> could add our own s390 code to QEMU to issue VFIO_DEVICE_SET_IRQS
>>>>> directly for an interpreted device, but I think would also need
>>>>> s390-specific changes to VFIO_DEVICE_SET_IRQS accommodate this (e.g.
>>>>> maybe something like a VFIO_IRQ_SET_DATA_S390AEN where we can then
>>>>> specify the aen information in vfio_irq_set.data -- or something 
>>>>> else I
>>>>
>>>> Hi,
>>>>
>>>> yes this in VFIO_DEVICE_SET_IRQS is what I think should be done.
>>>>
>>>>> haven't though of yet) -- I can try to look at this some more and 
>>>>> see if
>>>>> I get a good idea.
>>>>
>>>>
>>>> I understood that the demand was concerning the IOMMU but I may be 
>>>> wrong.
> 
> The IOMMU was an issue, but the request to move the ioctl out of vfio to 
> kvm was specifically because these ioctl operations were only relevant 
> for VMs and are not applicable to vfio uses cases outside of 
> virtualization.
> 
> https://lore.kernel.org/kvm/20220208185141.GH4160@nvidia.com/

I absolutely agree that KVM specific handling should go through KVM fd.
But as I say here under, AEN is not KVM specific but device specific.
Instruction interpretation is KVM specific.
see later---v

> 
>>>> For my opinion, the handling of AEN is not specific to KVM but specific
>>>> to the device, for example the code should be the same if Z ever decide
>>>> to use XEN or another hypervizor, except for the GISA part but this 
>>>> part
>>>> is already implemented in KVM in a way it can be used from a device 
>>>> like
>>>> in VFIO AP.
> 
> 
> Fundamentally, these operations are valid only when you have _both_ a 
> virtual machine and vfio device.  (Yes, you could swap in a new 
> hypervisor with a new GISA implementation, but at the end of it the 
> hypervisor must still provide the GISA designation for this to work)
> 
> If fh lookup is a concern, one idea that Jason floated was passing the 
> vfio device fd as an argument to the kvm ioctl (so pass this down on a 
> kvm ioctl from userspace instead of a fh) and then using a new vfio 
> external API to get the relevant device from the provided fd.
> 
> https://lore.kernel.org/kvm/20220208195117.GI4160@nvidia.com/

^------
This looks like a wrong architecture to me.

If something is used to virtualize the I/O of a device it should go 
through the device VFIO fd.

If we need a new VFIO external API why not using an extension of the 
VFIO_DEVICE_SET_IRQS and use directly the VFIO device to setup interrupts?

see following ----v

> 
>>>>
>>>> @Alex, what do you think?
>>>>
>>>> Regards,
>>>> Pierre
>>>>
>>>
>>> As I understand it the question isn't if it is specific to KVM but
>>> rather if it is specific to virtualization. As vfio-pci is also used
>>> for non virtualization purposes such as with DPDK/SPDK or a fully
>>> emulating QEMU, it should only be in VFIO if it is relevant for these
>>> kinds of user-space PCI accesses too. I'm not an AEN expert but as I
>>> understand it, this does forwarding interrupts into a SIE context which
>>> only makes sense for virtualization not for general user-space PCI.
> 
> Right, AEN forwarding is only relevant for virtual machines.
> 
>>>
>>
>> Being in VFIO kernel part does not mean that this part should be 
>> called from any user of VFIO in userland.
>> That is a reason why I did propose an extension and not using the 
>> current implementation of VFIO_DEVICE_SET_IRQS as is.
>>
>> The reason behind is that the AEN hardware handling is device 
>> specific: we need the Function Handle to program AEN.
> 
> You also need the GISA designation which is provided by the kvm or you 
> also can't program AEN.  So you ultimately need both a function handle 
> that is 'owned' by the device (vfio device fd) and the GISA designation 
> that is 'owned' by kvm (kvm fd).  So there are 2 different "owning" fds 
> involved.

Yes GISA is a host structure, not device specific but guest specific and 
exist very soon during the guest creation, there should be no problem to 
retrieve it from a VFIO device IOTCL.

> 
>>
>> If the API is through KVM which is device agnostic the implementation 
>> in KVM has to search through the system to find the device being 
>> handled to apply AEN on it.
> 
> See comment above about instead passing the vfio device fd.
> 
>>
>> This not the logical way for me and it is a potential source of 
>> problems for future extensions.
>>
> 
> 
>
^------

There are three different things to modify for the Z-guest to use VFIO:
- IOMMU
- device IRQ
- instruction interpretation, feature negociation

For my opinion only the last one should go directly through the KVM fd.

This should be possible for all architectures.
If it is not possible for Z, the failing path must be adapted it should 
not go through another path.

Giving the right IRQ information to the host can be done with a 
dedicated IOCTL through the VFIO device fd, just like we need an 
extension in the other direction to retrieve the Z specific capabilities.

I am quite sure that other architectures will need some specificity too 
for the interrupt or IOMMU handling in the future with increasing 
implementation of virtualization in the firmware.

Having a dedicated IOCTL command means it can be called from QEMU and 
for guest virtualizuation only then let unused for other userland access.


Regards,
Pierre


-- 
Pierre Morel
IBM Lab Boeblingen