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@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.
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
