Re: [PATCH v4 08/10] vfio/pci: Enable peer-to-peer DMA transactions by default

[Leon Romanovsky <leon@kernel.org>]

On Mon, Sep 29, 2025 at 03:17:45PM -0600, Alex Williamson wrote:
> On Sun, 28 Sep 2025 17:50:18 +0300
> Leon Romanovsky <leon@kernel.org> wrote:
> 
> > From: Leon Romanovsky <leonro@nvidia.com>
> > 
> > Make sure that all VFIO PCI devices have peer-to-peer capabilities
> > enables, so we would be able to export their MMIO memory through DMABUF,
> > 
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> >  drivers/vfio/pci/vfio_pci_core.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index 7dcf5439dedc..608af135308e 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -28,6 +28,9 @@
> >  #include <linux/nospec.h>
> >  #include <linux/sched/mm.h>
> >  #include <linux/iommufd.h>
> > +#ifdef CONFIG_VFIO_PCI_DMABUF
> > +#include <linux/pci-p2pdma.h>
> > +#endif
> >  #if IS_ENABLED(CONFIG_EEH)
> >  #include <asm/eeh.h>
> >  #endif
> > @@ -2085,6 +2088,7 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
> >  {
> >  	struct vfio_pci_core_device *vdev =
> >  		container_of(core_vdev, struct vfio_pci_core_device, vdev);
> > +	int __maybe_unused ret;
> >  
> >  	vdev->pdev = to_pci_dev(core_vdev->dev);
> >  	vdev->irq_type = VFIO_PCI_NUM_IRQS;
> > @@ -2094,6 +2098,11 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev)
> >  	INIT_LIST_HEAD(&vdev->dummy_resources_list);
> >  	INIT_LIST_HEAD(&vdev->ioeventfds_list);
> >  	INIT_LIST_HEAD(&vdev->sriov_pfs_item);
> > +#ifdef CONFIG_VFIO_PCI_DMABUF
> > +	ret = pcim_p2pdma_init(vdev->pdev);
> > +	if (ret)
> > +		return ret;
> > +#endif
> >  	init_rwsem(&vdev->memory_lock);
> >  	xa_init(&vdev->ctx);
> >  
> 
> What breaks if we don't test the return value and remove all the
> #ifdefs?  The feature call should fail if we don't have a provider but
> that seems more robust than failing to register the device.  Thanks,

pcim_p2pdma_init() fails if memory allocation fails, which is worth to check.
Such failure will most likely cause to non-working vfio-pci module anyway,
as failure in pcim_p2pdma_init() will trigger OOM. It is better to fail early
and help for the system to recover from OOM, instead of delaying to the
next failure while trying to load vfio-pci.

CONFIG_VFIO_PCI_DMABUF is mostly for next line "INIT_LIST_HEAD(&vdev->dmabufs);"
from the following patch. Because that pcim_p2pdma_init() and dmabufs list are
coupled, I put CONFIG_VFIO_PCI_DMABUF on both of them.

Thanks

> 
> Alex
> 
>