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

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Dec 30, 2021 at 01:34:27PM +0800, Lu Baolu wrote:
> 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.

It looks like that happens in device_dma_configure(), not
pci_dma_configure().

> > > 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@linux.intel.com>
> > > ---
> > >   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.

Yes.  But you didn't answer the question of whether this must be done
by a new flag in struct device_driver, or whether it could be done by
having these few VFIO and "VFIO-approved" (whatever that means)
drivers call a new interface.

I was speculating that maybe the DMA ownership claiming must be done
*before* the driver's .probe() method?  If so, that would require a
new flag.  But I don't know whether that's the case.  If DMA
ownership could be claimed by the .probe() method, we wouldn't need
the new flag in struct device_driver.

Bjorn