Re: [PATCH 1/3] vfio/pci: Set VFIO_PCI_OFFSET_SHIFT to 48

[Alex Williamson <alex@shazbot.org>]

On Thu, 12 Feb 2026 15:02:15 +0100
Julian Ruess <julianr@linux.ibm.com> wrote:

> Extend VFIO_PCI_OFFSET_SHIFT to 48 to use the vfio-pci
> VFIO_PCI_OFFSET_TO_INDEX() mechanism with the 256 TiB pseudo-BAR 0 of
> the ISM device on s390. This bar is never mapped.

Why does the entirety of vfio-pci need to adapt to the BAR requirements
of this device?  There's a variant driver included here that implements
its own read/write handlers and doesn't support mmap, so you're already
halfway there to implement your own region offsets independent of the
conventions of the rest of vfio-pci.  Thanks,

Alex
 
> Acked-by: Alexandra Winter <wintera@linux.ibm.com>
> Signed-off-by: Julian Ruess <julianr@linux.ibm.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c | 4 ++--
>  include/linux/vfio_pci_core.h    | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 3a11e6f450f70105f17a3a621520c195d99e0671..3d70bf6668c7a69c4b46674195954d1ada662006 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1647,7 +1647,7 @@ static unsigned long vma_to_pfn(struct vm_area_struct *vma)
>  	u64 pgoff;
>  
>  	pgoff = vma->vm_pgoff &
> -		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
> +		((1UL << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
>  
>  	return (pci_resource_start(vdev->pdev, index) >> PAGE_SHIFT) + pgoff;
>  }
> @@ -1751,7 +1751,7 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma
>  	phys_len = PAGE_ALIGN(pci_resource_len(pdev, index));
>  	req_len = vma->vm_end - vma->vm_start;
>  	pgoff = vma->vm_pgoff &
> -		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
> +		((1UL << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
>  	req_start = pgoff << PAGE_SHIFT;
>  
>  	if (req_start + req_len > phys_len)
> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> index 1ac86896875cf5c9b5cc8ef25fae8bbd4394de05..12781707f086a330161990dc3579ec0d75887da8 100644
> --- a/include/linux/vfio_pci_core.h
> +++ b/include/linux/vfio_pci_core.h
> @@ -20,7 +20,7 @@
>  #ifndef VFIO_PCI_CORE_H
>  #define VFIO_PCI_CORE_H
>  
> -#define VFIO_PCI_OFFSET_SHIFT   40
> +#define VFIO_PCI_OFFSET_SHIFT   48
>  #define VFIO_PCI_OFFSET_TO_INDEX(off)	(off >> VFIO_PCI_OFFSET_SHIFT)
>  #define VFIO_PCI_INDEX_TO_OFFSET(index)	((u64)(index) << VFIO_PCI_OFFSET_SHIFT)
>  #define VFIO_PCI_OFFSET_MASK	(((u64)(1) << VFIO_PCI_OFFSET_SHIFT) - 1)
>