Re: [PATCH 4/4] vfio-pci/zdev: Introduce the zPCI I/O vfio region

[Matthew Rosato]

On 1/21/21 5:01 AM, Niklas Schnelle wrote:
> On 1/19/21 9:02 PM, Matthew Rosato wrote:
> > Some s390 PCI devices (e.g. ISM) perform I/O operations that have very
> > specific requirements in terms of alignment as well as the patterns in
> > which the data is read/written. Allowing these to proceed through the
> > typical vfio_pci_bar_rw path will cause them to be broken in up in such a
> > way that these requirements can't be guaranteed. In addition, ISM devices
> > do not support the MIO codepaths that might be triggered on vfio I/O coming
> > from userspace; we must be able to ensure that these devices use the
> > non-MIO instructions.  To facilitate this, provide a new vfio region by
> > which non-MIO instructions can be passed directly to the host kernel s390
> > PCI layer, to be reliably issued as non-MIO instructions.
> >
> > This patch introduces the new vfio VFIO_REGION_SUBTYPE_IBM_ZPCI_IO region
> > and implements the ability to pass PCISTB and PCILG instructions over it,
> > as these are what is required for ISM devices.
> >
> > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> > ---
> >   drivers/vfio/pci/vfio_pci.c         |   8 ++
> >   drivers/vfio/pci/vfio_pci_private.h |   6 ++
> >   drivers/vfio/pci/vfio_pci_zdev.c    | 158 ++++++++++++++++++++++++++++++++++++
> >   include/uapi/linux/vfio.h           |   4 +
> >   include/uapi/linux/vfio_zdev.h      |  33 ++++++++
> >   5 files changed, 209 insertions(+)
>
> Related to the discussion on the QEMU side, if we have a check
> to make sure this is only used for ISM, then this patch should
> make that clear in its wording and also in the paths
> (drivers/vfio/pci/vfio_pci_ism.c instead of vfio_pci_zdev.c.)
> This also has precedent with the region for IGD in
> drivers/vfio/pci/vfio_pci_igd.c.

This is a fair point, but just to tie up threads here -- the QEMU 
discussion has since moved towards making the use-case less restrictive 
rather than ISM-only (though getting ISM working is still the motivating 
factor here).  So as such I think I'd prefer to keep this in 
vfio_pci_zdev.c as other hardware could use the region if they meet the 
necessary criteria.