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

[Farhan Ali <alifm@linux.ibm.com>]

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?

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;
>>