linux-kernel - Re: [PATCH v9 10/21] vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV

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Date:   Wed, 8 Jun 2022 08:19:25 +0200
From:   Thomas Huth <thuth@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>, linux-s390@...r.kernel.org
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...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, pasic@...ux.ibm.com,
        pbonzini@...hat.com, corbet@....net, jgg@...dia.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v9 10/21] vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV_KVM

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@...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 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).

So if you want to make this selectable by the user, I think you have to put 
some more #ifdefs in the following patches.
But if this was not meant to be selectable by the user, I think it should 
not get a help text and rather be selected by the KVM switch in 
arch/s390/kvm/Kconfig instead of having a "default y".

  Thomas