Re: [PATCH v4 0/3] vfio/pci: Introduce vfio_pci driver for ISM devices

[Alex Williamson <alex@shazbot.org>]

On Mon, 16 Mar 2026 13:33:04 +0100
"Julian Ruess" <julianr@linux.ibm.com> wrote:

> On Fri Mar 13, 2026 at 4:41 PM CET, Alex Williamson wrote:
> > On Fri, 13 Mar 2026 15:40:27 +0100
> > Julian Ruess <julianr@linux.ibm.com> wrote:
> >  
> >> Hi all,
> >> 
> >> This series adds a vfio_pci variant driver for the s390-specific
> >> Internal Shared Memory (ISM) devices used for inter-VM communication
> >> including SMC-D.
> >> 
> >> This is a prerequisite for an in-development open-source user space
> >> driver stack that will allow to use ISM devices to provide remote
> >> console and block device functionality. This stack will be part of
> >> s390-tools.
> >> 
> >> This driver would also allow QEMU to mediate access to an ISM device,
> >> enabling a form of PCI pass-through even for guests whose hardware
> >> cannot directly execute PCI accesses, such as nested guests.
> >> 
> >> On s390, kernel primitives such as ioread() and iowrite() are switched
> >> over from function handle based PCI load/stores instructions to PCI
> >> memory-I/O (MIO) loads/stores when these are available and not
> >> explicitly disabled. Since these instructions cannot be used with ISM
> >> devices, ensure that classic function handle-based PCI instructions are
> >> used instead.
> >> 
> >> The driver is still required even when MIO instructions are disabled, as
> >> the ISM device relies on the PCI store‑block (PCISTB) instruction to
> >> perform write operations.
> >> 
> >> Thank you,
> >> Julian
> >> 
> >> Signed-off-by: Julian Ruess <julianr@linux.ibm.com>
> >> ---
> >> Changes in v4:
> >> - Fix bug with < 8 byte reads. For code simplicity, only support 8 byte reads.  
> >
> > Does the ISM device define sub-8-byte accesses as valid?  It looks like
> > if pread() doesn't return the desired size QEMU will fill the return
> > with -1.  Unless such accesses are classified as undefined by ISM,
> > doesn't that suggest a potential data corruption issue to the guest
> > driver?  Thanks,
> >
> > Alex  
> 
> Hi Alex,
> 
> thanks for the quick feedback!
> 
> We are currently developing this extension for a non‑QEMU vfio user space
> driver. Reads smaller than 8 bytes are theoretically valid, but they are not
> used by this driver nor the existing in-kernel driver at the moment. We could
> extend this in the future if needed.
> 
> vfio‑pci based PCI pass-through of the ISM device is already possible without
> this extension. In that case, the ISM driver in the guest kernel accesses the
> BARs directly through hardware virtualization, without using the new access
> routines provided by this variant driver.

Hi Julian,

The cover letter argues a secondary use case with QEMU, especially in a
nested environment.  The ISM range appears to be an interface to a
variety of device types, console and block are noted.  It's also noted
in the implementation that the z/Architecture allows sub-8-byte access.

I think we need to be cautious that the existence of this driver makes
it available for use with QEMU and other VMMs.  In the case of QEMU
vfio_region_ops will allow single-byte access by default.

The restricted access width is positioned as a simplification here, but
it needs to be evaluated against all the use cases.  Unless we're 100%
sure none of those use cases rely on sub-8-byte accesses, we might be
setting ourselves up for hacks later to fix or detect partial access
support.

I'll leave it to IBM folks to determine if this is indeed a
simplification for long term support of all use cases and not a short
term fix for the short term use case.  Thanks,

Alex