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Date:   Tue, 18 Jan 2022 18:45:04 +0100
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Subject: Re: [PATCH v2 23/30] vfio/pci: re-introduce CONFIG_VFIO_PCI_ZDEV
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To:     Matthew Rosato <mjrosato@linux.ibm.com>, linux-s390@vger.kernel.org
Cc:     alex.williamson@redhat.com, cohuck@redhat.com,
        schnelle@linux.ibm.com, farman@linux.ibm.com,
        borntraeger@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
        vneethv@linux.ibm.com, oberpar@linux.ibm.com, freude@linux.ibm.com,
        thuth@redhat.com, pasic@linux.ibm.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220114203145.242984-1-mjrosato@linux.ibm.com>
 <20220114203145.242984-24-mjrosato@linux.ibm.com>
 <1ea61cf3-65b2-87ec-55b4-7dfa5f623d15@linux.ibm.com>
 <e1cd6368-bb1a-1a4d-df83-8190524b9a4d@linux.ibm.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
In-Reply-To: <e1cd6368-bb1a-1a4d-df83-8190524b9a4d@linux.ibm.com>
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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?

-- 
Pierre Morel
IBM Lab Boeblingen