Re: [PATCH v1] vfio/pci: Avoid mapping BARs for devices with non-mappable BARs

[Alex Williamson <alex@shazbot.org>]

On Wed, 29 Jul 2026 13:28:46 -0700
Farhan Ali <alifm@linux.ibm.com> wrote:

> On 7/29/2026 12:41 PM, Matthew Rosato wrote:
> > On 7/29/26 2:11 PM, Farhan Ali wrote:  
> >> vfio_pci_core_map_bars() calls pci_iomap() to set up BAR resources, but not
> >> all devices support having their BARs mapped by the CPU. The
> >> non_mappable_bars flag indicates that a PCI device's BARs cannot be
> >> accessed by the CPU. The ISM device on s390 is one such device. The BAR
> >> size for an ISM device is 256 TiB, and attempting to map the BAR will lead
> >> to warnings:
> >>
> >> vmalloc_node_range for size 281474976714752 failed: Address range
> >> restricted to 0x2110bab00000 - 0x21903ab00000
> >>
> >> Use pdev->non_mappable_bars to skip pci_iomap() for such devices. This flag
> >> is set by the PCI core at enumeration time and already serves the same
> >> purpose in vfio_pci_probe_mmaps().
> >>
> >> Fixes: 05f2a68b407a ("vfio/pci: Set up BAR resources and maps in vfio_pci_core_enable()")
> >> Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> >> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> >> ---
> >>   drivers/vfio/pci/vfio_pci_core.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> >> index 3f11a9624b9c..6a184588ff23 100644
> >> --- a/drivers/vfio/pci/vfio_pci_core.c
> >> +++ b/drivers/vfio/pci/vfio_pci_core.c
> >> @@ -554,6 +554,9 @@ static void vfio_pci_core_map_bars(struct vfio_pci_core_device *vdev)
> >>   
> >>   		vdev->barmap[bar] = IOMEM_ERR_PTR(-ENODEV);
> >>   
> >> +		if (pdev->non_mappable_bars)
> >> +			continue;
> >> +  
> > This would work for the ISM case at least, but I wonder: should we check
> > vdev->bar_mmap_supported[bar] instead?
> >
> > My question boils down to: do we still want messages for some of the
> > cases where we set vdev->bar_mmap_supported[bar] = false in
> > vfio_pci_probe_mmaps()?  
> 
> AFAIU vfio_pci_probe_mmaps() is only called in 
> vfio_pci_core_finish_enable(). Since vfio_pci_core_map_bars() is called 
> in vfio_pci_core_enable(), and before vfio_pci_core_finish_enable(), 
> bar_mmap_supported would be false here for all devices. So I don't think 
> it would work here, unless I missed something?

Also IO Port and sub-page MMIO BARs are things that do exist.  Thanks,

Alex