linux-kernel - Re: [PATCH v4 4/4] vfio: pci: Using a device region to retrieve zPCI information

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Message-ID: <20190920162607.16198c92.cohuck@redhat.com>
Date:   Fri, 20 Sep 2019 16:26:07 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     sebott@...ux.ibm.com, gerald.schaefer@...ibm.com,
        pasic@...ux.ibm.com, borntraeger@...ibm.com, walling@...ux.ibm.com,
        linux-s390@...r.kernel.org, iommu@...ts.linux-foundation.org,
        joro@...tes.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, kvm@...r.kernel.org,
        heiko.carstens@...ibm.com, robin.murphy@....com, gor@...ux.ibm.com,
        pmorel@...ux.ibm.com
Subject: Re: [PATCH v4 4/4] vfio: pci: Using a device region to retrieve
 zPCI information

On Thu, 19 Sep 2019 16:57:10 -0400
Matthew Rosato <mjrosato@...ux.ibm.com> wrote:

> On 9/19/19 11:25 AM, Cornelia Huck wrote:
> > On Fri,  6 Sep 2019 20:13:51 -0400
> > Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
> >   
> >> From: Pierre Morel <pmorel@...ux.ibm.com>
> >>
> >> We define a new configuration entry for VFIO/PCI, VFIO_PCI_ZDEV
> >>
> >> When the VFIO_PCI_ZDEV feature is configured we initialize
> >> a new device region, VFIO_REGION_SUBTYPE_ZDEV_CLP, to hold
> >> the information from the ZPCI device the use
> >>
> >> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
> >> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> >> ---
> >>  drivers/vfio/pci/Kconfig            |  7 +++
> >>  drivers/vfio/pci/Makefile           |  1 +
> >>  drivers/vfio/pci/vfio_pci.c         |  9 ++++
> >>  drivers/vfio/pci/vfio_pci_private.h | 10 +++++
> >>  drivers/vfio/pci/vfio_pci_zdev.c    | 85 +++++++++++++++++++++++++++++++++++++
> >>  5 files changed, 112 insertions(+)
> >>  create mode 100644 drivers/vfio/pci/vfio_pci_zdev.c
> >>
> >> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> >> index ac3c1dd..d4562a8 100644
> >> --- a/drivers/vfio/pci/Kconfig
> >> +++ b/drivers/vfio/pci/Kconfig
> >> @@ -45,3 +45,10 @@ config VFIO_PCI_NVLINK2
> >>  	depends on VFIO_PCI && PPC_POWERNV
> >>  	help
> >>  	  VFIO PCI support for P9 Witherspoon machine with NVIDIA V100 GPUs
> >> +
> >> +config VFIO_PCI_ZDEV
> >> +	bool "VFIO PCI Generic for ZPCI devices"
> >> +	depends on VFIO_PCI && S390
> >> +	default y
> >> +	help
> >> +	  VFIO PCI support for S390 Z-PCI devices  
> >   
> >>From that description, I'd have no idea whether I'd want that or not.  
> > Is there any downside to enabling it?
> >   
> 
> :) Not really, you're just getting information from the hardware vs
> using hard-coded defaults.  The only reason I could think of to turn it
> off would be if you wanted/needed to restore this hard-coded behavior.

I'm not really sure whether that's worth adding a Kconfig switch for.
Won't older versions simply ignore the new region anyway?

Also, I don't think we have any migration compatibility issues, as
vfio-pci devices are not (yet) migrateable anyway.

> 
> bool "VFIO PCI support for generic ZPCI devices" ?

"Support zPCI-specific configuration for VFIO PCI" ?

> 
> "Support for sharing ZPCI hardware device information between the host
> and guests." ?

"Enabling this options exposes a region containing hardware
configuration for zPCI devices. This enables userspace (e.g. QEMU) to
supply proper configuration values instead of hard-coded defaults for
zPCI devices passed through via VFIO on s390.

Say Y here."

?