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

[Alex Williamson]

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