Re: [PATCH v7 06/11] PCI: portdrv: Set driver_managed_dma

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Feb 28, 2022 at 08:50:51AM +0800, Lu Baolu wrote:
> If a switch lacks ACS P2P Request Redirect, a device below the switch can
> bypass the IOMMU and DMA directly to other devices below the switch, so
> all the downstream devices must be in the same IOMMU group as the switch
> itself.
> 
> The existing VFIO framework allows the portdrv driver to be bound to the
> bridge while its downstream devices are assigned to user space. The
> pci_dma_configure() marks the IOMMU group as containing only devices
> with kernel drivers that manage DMA. Avoid this default behavior for the
> portdrv driver in order for compatibility with the current VFIO usage.

It would be nice to explicitly say here how we can look at portdrv
(and pci_stub) and conclude that ".driver_managed_dma = true" is safe.

Otherwise I won't know what kind of future change to portdrv might
make it unsafe.

> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Suggested-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
>  drivers/pci/pcie/portdrv_pci.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
> index 35eca6277a96..6b2adb678c21 100644
> --- a/drivers/pci/pcie/portdrv_pci.c
> +++ b/drivers/pci/pcie/portdrv_pci.c
> @@ -202,6 +202,8 @@ static struct pci_driver pcie_portdriver = {
>  
>  	.err_handler	= &pcie_portdrv_err_handler,
>  
> +	.driver_managed_dma = true,
> +
>  	.driver.pm	= PCIE_PORTDRV_PM_OPS,
>  };
>  
> -- 
> 2.25.1
> 
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