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

[Farhan Ali]

On 7/29/2026 2:50 PM, Alex Williamson wrote:
> On Wed, 29 Jul 2026 14:32:45 -0700
> Farhan Ali <alifm@xxxxxxxxxxxxx> wrote:
>
> > On 7/29/2026 1:36 PM, Alex Williamson wrote:
> > > On Wed, 29 Jul 2026 13:28:46 -0700
> > > Farhan Ali <alifm@xxxxxxxxxxxxx> 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@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?
> > > Also IO Port and sub-page MMIO BARs are things that do exist.  Thanks,
> > Just to clarify, are you suggesting we expand this check to also avoid
> > mapping IO port and sub-page MMIO BARs?
> Sorry, no, I'm not.  I think we're conflating that the barmap is
> related to mmap access.  The barmap itself is holding the iomap of the
> BAR, used for read/write.  The only real relation to the mmap is that
> we request the resource via this path as well.
>
> Therefore not only is the ordering of setting up bar_mmap_supported
> wrong, it's flagging entirely the wrong thing here and keying on it
> would entirely break IO port and sub-page MMIO BAR access.  Thanks,
>
> Alex

Hi Alex,

I wanted some guidance on how we should proceed with this patch? The 
warning messages are a regression on s390 for ISM devices, so we would 
like to fix it.

Thanks

Farhan