Re: [PATCH 1/1] iommu/vt-d: Allow zero SAGAW if second-stage not supported

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

On 2023/3/24 12:49, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Saturday, March 18, 2023 10:48 AM
>>
>> The VT-d spec states (section 11.4.2) that hardware implementations
>> reporting second-stage translation support (SSTS) field as Clear also
>> report the SAGAW field as 0. Reflect this in the sanity check of
>> alloc_iommu().
>>
>> Fixes: 792fb43ce2c9 ("iommu/vt-d: Enable Intel IOMMU scalable mode by
>> default")
>> Suggested-by: Raghunathan Srinivasan <raghunathan.srinivasan@intel.com>
>> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>>   drivers/iommu/intel/dmar.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
>> index 6acfe879589c..23828d189c2a 100644
>> --- a/drivers/iommu/intel/dmar.c
>> +++ b/drivers/iommu/intel/dmar.c
>> @@ -1071,7 +1071,8 @@ static int alloc_iommu(struct dmar_drhd_unit
>> *drhd)
>>   	}
>>
>>   	err = -EINVAL;
>> -	if (cap_sagaw(iommu->cap) == 0) {
>> +	if (!cap_sagaw(iommu->cap) &&
>> +	    (!ecap_smts(iommu->ecap) || ecap_slts(iommu->ecap))) {
>>   		pr_info("%s: No supported address widths. Not attempting
>> DMA translation.\n",
>>   			iommu->name);
>>   		drhd->ignored = 1;
> 
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> 
> btw I wonder whether it's cleaner to record separate agaw values for
> stage1/stage2 instead of picking a minimal set from both in
> __iommu_calculate_sagaw().

That's better. The agaw could be picked according to which stage the
domain is used for translation.

Best regards,
baolu