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

[Lu Baolu]

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