linux-kernel - Re: [PATCH v4 4/4] vfio: pci: Using a device region to retrieve zPCI information

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Date:   Fri, 20 Sep 2019 11:53:41 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     sebott@...ux.ibm.com, gerald.schaefer@...ibm.com,
        pasic@...ux.ibm.com, borntraeger@...ibm.com, walling@...ux.ibm.com,
        linux-s390@...r.kernel.org, iommu@...ts.linux-foundation.org,
        joro@...tes.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, kvm@...r.kernel.org,
        heiko.carstens@...ibm.com, robin.murphy@....com, gor@...ux.ibm.com,
        pmorel@...ux.ibm.com
Subject: Re: [PATCH v4 4/4] vfio: pci: Using a device region to retrieve zPCI
 information

On 9/20/19 10:26 AM, Cornelia Huck wrote:
> On Thu, 19 Sep 2019 16:57:10 -0400
> Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
> 
>> On 9/19/19 11:25 AM, Cornelia Huck wrote:
>>> On Fri,  6 Sep 2019 20:13:51 -0400
>>> Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
>>>   
>>>> From: Pierre Morel <pmorel@...ux.ibm.com>
>>>>
>>>> We define a new configuration entry for VFIO/PCI, VFIO_PCI_ZDEV
>>>>
>>>> When the VFIO_PCI_ZDEV feature is configured we initialize
>>>> a new device region, VFIO_REGION_SUBTYPE_ZDEV_CLP, to hold
>>>> the information from the ZPCI device the use
>>>>
>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
>>>> ---
>>>>  drivers/vfio/pci/Kconfig            |  7 +++
>>>>  drivers/vfio/pci/Makefile           |  1 +
>>>>  drivers/vfio/pci/vfio_pci.c         |  9 ++++
>>>>  drivers/vfio/pci/vfio_pci_private.h | 10 +++++
>>>>  drivers/vfio/pci/vfio_pci_zdev.c    | 85 +++++++++++++++++++++++++++++++++++++
>>>>  5 files changed, 112 insertions(+)
>>>>  create mode 100644 drivers/vfio/pci/vfio_pci_zdev.c
>>>>
>>>> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
>>>> index ac3c1dd..d4562a8 100644
>>>> --- a/drivers/vfio/pci/Kconfig
>>>> +++ b/drivers/vfio/pci/Kconfig
>>>> @@ -45,3 +45,10 @@ config VFIO_PCI_NVLINK2
>>>>  	depends on VFIO_PCI && PPC_POWERNV
>>>>  	help
>>>>  	  VFIO PCI support for P9 Witherspoon machine with NVIDIA V100 GPUs
>>>> +
>>>> +config VFIO_PCI_ZDEV
>>>> +	bool "VFIO PCI Generic for ZPCI devices"
>>>> +	depends on VFIO_PCI && S390
>>>> +	default y
>>>> +	help
>>>> +	  VFIO PCI support for S390 Z-PCI devices  
>>>   
>>> >From that description, I'd have no idea whether I'd want that or not.  
>>> Is there any downside to enabling it?
>>>   
>>
>> :) Not really, you're just getting information from the hardware vs
>> using hard-coded defaults.  The only reason I could think of to turn it
>> off would be if you wanted/needed to restore this hard-coded behavior.
> 
> I'm not really sure whether that's worth adding a Kconfig switch for.
> Won't older versions simply ignore the new region anyway?
> 

Yes, you have a point here...  This switch showed up in v3 of this
series when Pierre changed to using a region to pass this info and I
haven't yet found a 'why' he decided to add the Kconfig switch.  If I
can't convince myself of a reason to keep it, I'll just remove it from
the next version.

> Also, I don't think we have any migration compatibility issues, as
> vfio-pci devices are not (yet) migrateable anyway.
> 
>>
>> bool "VFIO PCI support for generic ZPCI devices" ?
> 
> "Support zPCI-specific configuration for VFIO PCI" ?
> 
>>
>> "Support for sharing ZPCI hardware device information between the host
>> and guests." ?
> 
> "Enabling this options exposes a region containing hardware
> configuration for zPCI devices. This enables userspace (e.g. QEMU) to
> supply proper configuration values instead of hard-coded defaults for
> zPCI devices passed through via VFIO on s390.
> 
> Say Y here."
> 
> ?
>

Your descriptions are much better - thanks for the feedback!