Re: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

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Hi Bjorn,

On 12/30/21 4:42 AM, Bjorn Helgaas wrote:
On Fri, Dec 17, 2021 at 02:36:58PM +0800, Lu Baolu wrote:
The pci_dma_configure() marks the iommu_group as containing only devices
with kernel drivers that manage DMA.

I'm looking at pci_dma_configure(), and I don't see the connection to
iommu_groups.

The 2nd patch "driver core: Set DMA ownership during driver bind/unbind"
sets all drivers' DMA to be kernel-managed by default except a few ones
which has a driver flag set. So by default, all iommu groups contains
only devices with kernel drivers managing DMA.


Avoid this default behavior for the
pci_stub because it does not program any DMA itself.  This allows the
pci_stub still able to be used by the admin to block driver binding after
applying the DMA ownership to vfio.


Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
---
  drivers/pci/pci-stub.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/drivers/pci/pci-stub.c b/drivers/pci/pci-stub.c
index e408099fea52..6324c68602b4 100644
--- a/drivers/pci/pci-stub.c
+++ b/drivers/pci/pci-stub.c
@@ -36,6 +36,9 @@ static struct pci_driver stub_driver = {
  	.name		= "pci-stub",
  	.id_table	= NULL,	/* only dynamic id's */
  	.probe		= pci_stub_probe,
+	.driver		= {
+		.suppress_auto_claim_dma_owner = true,

The new .suppress_auto_claim_dma_owner controls whether we call
iommu_device_set_dma_owner().  I guess you added
.suppress_auto_claim_dma_owner because iommu_device_set_dma_owner()
must be done *before* we call the driver's .probe() method?

As explained above, all drivers are set to kernel-managed dma by
default. For those vfio and vfio-approved drivers,
suppress_auto_claim_dma_owner is used to tell the driver core that "this
driver is attached to device for userspace assignment purpose, do not
claim it for kernel-management dma".


Otherwise, we could call some new interface from .probe() instead of
adding the flag to struct device_driver.

Most device drivers are of the kernel-managed DMA type. Only a few vfio
and vfio-approved drivers need to use this flag. That's the reason why
we claim kernel-managed DMA by default.


+	},
  };
static int __init pci_stub_init(void)
--
2.25.1


Best regards,
baolu



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