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

[Lu Baolu <baolu.lu@linux.intel.com>]

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@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.

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

Best regards,
baolu