linux-kernel - Re: [PATCH v2 23/30] vfio/pci: re-introduce CONFIG_VFIO_PCI

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Date:   Tue, 18 Jan 2022 13:05:33 -0500
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>, linux-s390@...r.kernel.org
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        vneethv@...ux.ibm.com, oberpar@...ux.ibm.com, freude@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 23/30] vfio/pci: re-introduce CONFIG_VFIO_PCI_ZDEV

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@...ux.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.