Re: [PATCH] PCI/P2PDMA: Avoid returning a provider for non_mappable_bars

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Tue, 2026-04-21 at 10:43 -0700, Matt Evans wrote:
> Extend pcim_p2pdma_provider()'s checks to exclude functions that have
> pdev->non_mappable_bars set.
> 
> Consumers such as VFIO were previously able to map these for access by
> the CPU or P2P.  Update the comment on non_mappable_bars to show it
> refers to any access, not just userspace CPU access.
> 
> Fixes: 372d6d1b8ae3c ("PCI/P2PDMA: Refactor to separate core P2P functionality from memory allocation")
> Signed-off-by: Matt Evans <mattev@meta.com>
> ---
> 
> This arises from Alex Williamson's suggestion to test
> non_mappable_bars when getting the provider, with discussion here:
> 
>  https://lore.kernel.org/kvm/20260415181623.1021090-1-mattev@meta.com/
> 
> The goal was to prevent a hole where VFIO could export DMABUFs for
> BARs marked non-mappable, and to fix for all users of the provider
> rather than just VFIO.  Alex observed that non_mappable_bars should be
> taken to mean BARs weren't usable by the CPU _or_ peers and,
> considering that, its comment about userspace access wasn't quite
> right.
> 
> 
>  drivers/pci/p2pdma.c | 3 ++-
>  include/linux/pci.h  | 2 +-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 7c898542af8d..4a783413f466 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -318,7 +318,8 @@ struct p2pdma_provider *pcim_p2pdma_provider(struct pci_dev *pdev, int bar)
>  {
>  	struct pci_p2pdma *p2p;
>  
> -	if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM))
> +	if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM) ||
> +	    pdev->non_mappable_bars)
>  		return NULL;
>  
>  	p2p = rcu_dereference_protected(pdev->p2pdma, 1);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 2c4454583c11..1e6802017d6b 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -508,7 +508,7 @@ struct pci_dev {
>  	unsigned int	no_command_memory:1;	/* No PCI_COMMAND_MEMORY */
>  	unsigned int	rom_bar_overlap:1;	/* ROM BAR disable broken */
>  	unsigned int	rom_attr_enabled:1;	/* Display of ROM attribute enabled? */
> -	unsigned int	non_mappable_bars:1;	/* BARs can't be mapped to user-space  */
> +	unsigned int	non_mappable_bars:1;	/* BARs can't be mapped by CPU or peers */
>  	pci_dev_flags_t dev_flags;
>  	atomic_t	enable_cnt;	/* pci_enable_device has been called */
>  

Sorry for the late reply. For s390's ISM device this definitely makes
sense. The BAR can indeed not be accessed by P2P and extending this
seems reasonable to me. 

Feel free to add my:

Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>

Thanks,
Niklas