Re: [PATCH v9 10/21] vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV_KVM

[Thomas Huth]

On 14/06/2022 10.56, Pierre Morel wrote:
> On 6/8/22 15:15, Matthew Rosato wrote:
> > On 6/8/22 2:19 AM, Thomas Huth wrote:
> > > On 06/06/2022 22.33, Matthew Rosato wrote:
> > > > The current contents of vfio-pci-zdev are today only useful in a KVM
> > > > environment; let's tie everything currently under vfio-pci-zdev to
> > > > this Kconfig statement and require KVM in this case, reducing complexity
> > > > (e.g. symbol lookups).
> > > >
> > > > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
>
> Reviewed-by: Pierre Morel <pmorel@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 4da1914425e1..f9d0c908e738 100644
> > > > --- a/drivers/vfio/pci/Kconfig
> > > > +++ b/drivers/vfio/pci/Kconfig
> > > > @@ -44,6 +44,17 @@ config VFIO_PCI_IGD
> > > >         To enable Intel IGD assignment through vfio-pci, say Y.
> > > >   endif
> > > > +config VFIO_PCI_ZDEV_KVM
> > > > +    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.
> > >
> > > Is it still possible to disable CONFIG_VFIO_PCI_ZDEV_KVM ? Looking at the 
> > > later patches (e.g. 20/21 where you call kvm_s390_pci_zpci_op() from 
> > > kvm-s390.c), it rather seems to me that it currently cannot be disabled 
> > > independently (as long as KVM is enabled).
> >
> > Yes, you can build with, for example, CONFIG_VFIO_PCI_ZDEV_KVM=n and 
> > CONFIG_KVM=m -- I tested it again just now.  The result is kvm and 
> > vfio-pci are built and vfio-pci works, but none of the vfio-pci-zdev 
> > extensions are available (including zPCI interpretation).
> >
> > This is accomplished via the placement of some IS_ENABLED checks.  Some 
> > calls (e.g. AEN init) are fenced by IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM).  
> > There are also some areas that are fenced off via a call to 
> > kvm_s390_pci_interp_allowed() which also includes an IS_ENABLED check 
> > along with checks for facility and cpu id.
> >
> > Using patch 20 as an example, KVM_CAP_S390_ZPCI_OP will always be reported 
> > as unavailable to userspace if CONFIG_VFIO_PCI_ZDEV_KVM=n due to the call 
> > to kvm_s390_pci_interp_allowed().  If userspace sends us the ioctl anyway, 
> > we will return -EINVAL because there is again a 
> > IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM) check before we read the ioctl args 
> > from userspace.
>
> Yes and the code will not be generated by the compiler in patch 20 after the 
> break if CONFIG_VFIO_PCI_ZDEV_KVM is not enabled.
>
> +    case KVM_S390_ZPCI_OP: {
> +        struct kvm_s390_zpci_op args;
> +
> +        r = -EINVAL;
> +        if (!IS_ENABLED(CONFIG_VFIO_PCI_ZDEV_KVM))
> +            break;
>
> Code not generated----v
>
> +        if (copy_from_user(&args, argp, sizeof(args))) {
> +            r = -EFAULT;
> +            break;
> +        }
> +        r = kvm_s390_pci_zpci_op(kvm, &args);
> +        break;
>
> ----------^

OK, good to know, thanks for the clarification!

 Thomas