Re: [PATCH v6 0/3] vfio/pci: s390: Fix issues preventing VFIO_PCI_MMAP=y for s390 and enable it

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2025-02-24 at 14:53 -0600, Bjorn Helgaas wrote:
> On Fri, Feb 14, 2025 at 02:10:51PM +0100, Niklas Schnelle wrote:
> > With the introduction of memory I/O (MIO) instructions enbaled in commit
> > 71ba41c9b1d9 ("s390/pci: provide support for MIO instructions") s390
> > gained support for direct user-space access to mapped PCI resources.
> > Even without those however user-space can access mapped PCI resources
> > via the s390 specific MMIO syscalls. There is thus nothing fundamentally
> > preventing s390 from supporting VFIO_PCI_MMAP, allowing user-space
> > drivers to access PCI resources without going through the pread()
> > interface. To actually enable VFIO_PCI_MMAP a few issues need fixing
> > however.
> > 
> > Firstly the s390 MMIO syscalls do not cause a page fault when
> > follow_pte() fails due to the page not being present. This breaks
> > vfio-pci's mmap() handling which lazily maps on first access.
> > 
> > Secondly on s390 there is a virtual PCI device called ISM which has
> > a few oddities. For one it claims to have a 256 TiB PCI BAR (not a typo)
> > which leads to any attempt to mmap() it fail with the following message:
> > 
> >     vmap allocation for size 281474976714752 failed: use vmalloc=<size> to increase size
> > 
> > Even if one tried to map this BAR only partially the mapping would not
> > be usable on systems with MIO support enabled. So just block mapping
> > BARs which don't fit between IOREMAP_START and IOREMAP_END. Solve this
> > by keeping the vfio-pci mmap() blocking behavior around for this
> > specific device via a PCI quirk and new pdev->non_mappable_bars
> > flag.
> > 
> > As noted by Alex Williamson With mmap() enabled in vfio-pci it makes
> > sense to also enable HAVE_PCI_MMAP with the same restriction for pdev->
> > non_mappable_bars. So this is added in patch 3 and I tested this with
> > another small test program.
> > 
> > Note:
> > For your convenience the code is also available in the tagged
> > b4/vfio_pci_mmap branch on my git.kernel.org site below:
> > https://git.kernel.org/pub/scm/linux/kernel/git/niks/linux.git/
> > 
> > Thanks,
> > Niklas
> > 
> > Link: https://lore.kernel.org/all/c5ba134a1d4f4465b5956027e6a4ea6f6beff969.camel@linux.ibm.com/
> > Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> > ---
> > Changes in v6:
> > - Add a patch to also enable PCI resource mmap() via sysfs and proc
> >   exlcluding pdev->non_mappable_bars devices (Alex Williamson)
> > - Added Acks
> > - Link to v5: https://lore.kernel.org/r/20250212-vfio_pci_mmap-v5-0-633ca5e056da@linux.ibm.com
> 
> I think the series would be more readable if patch 2/3 included all
> the core changes (adding pci_dev.non_mappable_bars, the 3/3
> pci-sysfs.c and proc.c changes to test it, and I suppose the similar
> vfio_pci_core.c change), and we moved all the s390 content from 2/3 to
> 3/3.

Maybe we could do the following:

1/3: As is

2/3: Introduces pdev->non_mappable_bars and the checks in vfio and
proc.c/pci-sysfs.c. To make the flag handle the vfio case with
VFIO_PCI_MMAP gone, a one-line change in s390 will set pdev-
>non_mappable_bars = 1 for all PCI devices.

3/3: Changes setting pdev->non_mappable_bars = 1 in s390 to only the
ISM device using the quirk handling and adds HAVE_PCI_MMAP.

Thanks,
Niklas