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

[Matthew Rosato]

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.

> 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).