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SubjectRe: [PATCH v7 06/11] PCI: portdrv: Set driver_managed_dma
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Hi Bjorn,

On 3/1/22 3:56 AM, Bjorn Helgaas wrote:
> 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.

Fair enough. We can add below words:

We achieve this by setting ".driver_managed_dma = true" in pci_driver
structure. It is safe because the portdrv driver meets below criteria:

- This driver doesn't use DMA, as you can't find any related calls like
pci_set_master() or any kernel DMA API (dma_map_*() and etc.).
- It doesn't use MMIO as you can't find ioremap() or similar calls. It's
tolerant to userspace possibly also touching the same MMIO registers
via P2P DMA access.

>
>> 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>

Thank you!

Best regards,
baolu

>
>> ---
>> 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
>>
>> _______________________________________________
>> iommu mailing list
>> iommu@lists.linux-foundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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