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

[Farhan Ali]

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@xxxxxxxxxxxxx>
> > Signed-off-by: Farhan Ali <alifm@xxxxxxxxxxxxx>
> > ---
> >   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?

Thanks

Farhan

> Or should the messages only print when we probed, thought the bar could
> be mmap'd (vdev->bar_mmap_supported[bar] == true), and now hit a failure?
>
> Thanks,
> Matt
>
> >   		if (!pci_resource_len(pdev, i))
> >   			continue;
> >