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

Matt Evans
2026-04-21T17:48:31Z

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 */

--
2.47.3