Re: [PATCH 1/1] iommu/vt-d: PRS isn't usable if PDS isn't supported

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

On 9/15/25 19:30, Joel Granados wrote:
> On Mon, Sep 15, 2025 at 02:29:46PM +0800, Lu Baolu wrote:
>> The specification, Section 7.10, "Software Steps to Drain Page Requests &
>> Responses," requires software to submit an Invalidation Wait Descriptor
>> (inv_wait_dsc) with the Page-request Drain (PD=1) flag set, along with
>> the Invalidation Wait Completion Status Write flag (SW=1). It then waits
>> for the Invalidation Wait Descriptor's completion.
>>
>> However, the PD field in the Invalidation Wait Descriptor is optional, as
>> stated in Section 6.5.2.9, "Invalidation Wait Descriptor":
>>
>> "Page-request Drain (PD): Remapping hardware implementations reporting
>>   Page-request draining as not supported (PDS = 0 in ECAP_REG) treat this
>>   field as reserved."
>>
>> This implies that if the IOMMU doesn't support the PDS capability, software
>> can't drain page requests and group responses as expected.
>>
>> Do not enable PCI/PRI if the IOMMU doesn't support PDS.
> 
> After giving the spec another look, this is probably the way to go.
> However the PDS also mentions that DT must be set. Should we check
> ecap_dev_iotlb_support(iommu->ecap)  as well?

It has already been checked.

>>
>> Reported-by: Joel Granados <joel.granados@kernel.org>
>> Closes: https://lore.kernel.org/r/20250909-jag-pds-v1-1-ad8cba0e494e@kernel.org
>> Fixes: 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>>   drivers/iommu/intel/iommu.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index 9c3ab9d9f69a..92759a8f8330 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -3812,7 +3812,7 @@ static struct iommu_device *intel_iommu_probe_device(struct device *dev)
>>   			}
>>   
>>   			if (info->ats_supported && ecap_prs(iommu->ecap) &&
>> -			    pci_pri_supported(pdev))
>> +			    ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
> Should this be
>   +			    ecap_dev_iotlb_support(iommu->ecap) && ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
> 
> ???

ecap_dev_iotlb_support() has already been checked as it's part of the
info->ats_supported check. The dependency chain is:
ecap_dev_iotlb_support() -> ats_support -> pri_support.

Thanks,
baolu