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

On Mon, 3 Aug 2026 09:39:19 -0700
Farhan Ali <[email protected]> wrote:

> On 7/29/2026 2:50 PM, Alex Williamson wrote:
> > On Wed, 29 Jul 2026 14:32:45 -0700
> > Farhan Ali <[email protected]> wrote:
> >  
> >> On 7/29/2026 1:36 PM, Alex Williamson wrote:  
> >>> On Wed, 29 Jul 2026 13:28:46 -0700
> >>> Farhan Ali <[email protected]> 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 <[email protected]>
> >>>>>> Signed-off-by: Farhan Ali <[email protected]>
> >>>>>> ---
> >>>>>>     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.

I think the original proposal is probably the correct one.  The
non_mmapable_bars flag doesn't restrict its application to specific BAR
types or access, at least not beyond "can't be mapped to CPU or peers."

If we can't map the BAR to the CPU, then we don't need to request the
region or perform the pci_iomap(), which is what I understand explodes
here.  Therefore we really only need to establish the errno in the
barmap here.

The bar_mmap_supported flag describes something else, whether the BAR
can be mapped into the user address space.  My intention was only to
point out that there are BARs that cannot be mapped to the user address
space because either they're not MMIO or we can't safely map the full
page, therefore bar_mmap_supported is an invalid test for whether we
should request the region or iomap the BAR.

Is there still a gap with the original proposal that I'm missing?
Thanks,

Alex