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

[Pierre Morel]

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@xxxxxxxxxxxxx>
> > > ---
> > >   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