Re: [PATCH v2 23/30] vfio/pci: re-introduce CONFIG_VFIO_PCI_ZDEV

Matthew Rosato
2022-01-18T18:05:42Z

On 1/18/22 12:45 PM, Pierre Morel wrote:
>
>
> On 1/18/22 18:32, Matthew Rosato wrote:
>> On 1/18/22 12:20 PM, Pierre Morel wrote:
>>>
>>>
>>> On 1/14/22 21:31, Matthew Rosato wrote:
>>>> This was previously removed as unnecessary; while that was true,
>>>> subsequent
>>>> changes will make KVM an additional required component for
>>>> vfio-pci-zdev.
>>>> Let's re-introduce CONFIG_VFIO_PCI_ZDEV as now there is actually a
>>>> reason
>>>> to say 'n' for it (when not planning to CONFIG_KVM).
>>>>
>>>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>>>> ---
>>>>   drivers/vfio/pci/Kconfig      | 11 +++++++++++
>>>>   drivers/vfio/pci/Makefile     |  2 +-
>>>>   include/linux/vfio_pci_core.h |  2 +-
>>>>   3 files changed, 13 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
>>>> index 860424ccda1b..fedd1d4cb592 100644
>>>> --- a/drivers/vfio/pci/Kconfig
>>>> +++ b/drivers/vfio/pci/Kconfig
>>>> @@ -42,5 +42,16 @@ config VFIO_PCI_IGD
>>>>         and LPC bridge config space.
>>>>         To enable Intel IGD assignment through vfio-pci, say Y.
>>>> +
>>>> +config VFIO_PCI_ZDEV
>>>> +    bool "VFIO PCI extensions for s390x KVM passthrough"
>>>> +    depends on S390 && KVM
>>>> +    default y
>>>> +    help
>>>> +      Support s390x-specific extensions to enable support for
>>>> enhancements
>>>> +      to KVM passthrough capabilities, such as interpretive
>>>> execution of
>>>> +      zPCI instructions.
>>>> +
>>>> +      To enable s390x KVM vfio-pci extensions, say Y.
>>>
>>> In several patches we check on CONFIG_PCI (14,15,16,17 and 22) but we
>>> may have PCI without VFIO_PCI, wouldn't it be a problem?
>>>
>>> Here we define a new CONFIG entry and I have two questions:
>>>
>>> 1- there is no dependency on VFIO_PCI while the functionality is
>>> obviously based on VFIO_PCI
>>
>> It's not obvious from this diff, but this 'config VFIO_PCI_ZDEV'
>> statement is within an 'if VFIO_PCI' statement, just like VFIO_PCI_IGD
>> above -- so the dependency is there.
>
> sorry, I remember now you already answered this to Christian last time.
>
>>
>>>
>>> 2- Wouldn't it be possible to use this item and the single condition
>>> for the different checks we need through the new VFIO interpretation
>>> functionality.
>>
>> Possibly, but 1) we'd have to make linking arch/s390/kvm/pci.o
>> dependent on CONFIG_VFIO_PCI instead of CONFIG_PCI in patch 14 and 2)
>> if the relationship between CONFIG_VFIO_PCI and CONFIG_PCI were to
>> ever change (though I don't see why it would..), we would be broken
>> because the symbols we are referencing really require CONFIG_PCI (as
>> they are located in s390 PCI).
>>
>
> Yes but VFIO_PCI_ZDEV depends on KVM, PCI and on VFIO_PCI
> Wouldn't a single config item for this new code be easier to manage and
> understand?
>

I guess my primary resistance is to abstracting/hiding the dependency.
Yes, userspace will never setup for zPCI interpretation without
CONFIG_VFIO_PCI{_ZDEV}, but that's not where the compilation dependency
is -- it's on CONFIG_PCI specifically.

But I guess on the other hand you could argue why even bother building
pci.o into kvm without CONFIG_VFIO_PCI_ZDEV as it will never be used.

OK, I will have a look at making this change. It will require a little
reorganization, at least moving this patch up before patch 14.