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

[Farhan Ali]

On 8/3/2026 12:54 PM, Alex Williamson wrote:
> On Mon, 3 Aug 2026 09:39:19 -0700
> Farhan Ali <alifm@xxxxxxxxxxxxx> wrote:
>
> > 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.
> 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.

Yes, its the call to pci_iomap() that leads to the warning.

> 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?

AFAICT this fix should be sufficient. I think Matt's question was to see 
if there were other cases for which bar_mmap_supported were set to 
false, that we need to consider here. But I will let him clarify to see 
if he has any concerns.

Thanks

Farhan