Re: [PATCH 2/4] iommu: Add PASID support for DMA mapping API users

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

On 12/9/21 9:56 AM, Tian, Kevin wrote:
>> From: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Sent: Thursday, December 9, 2021 2:50 AM
>>
>>> Can a device issue DMA requests with PASID even there's no system
>> IOMMU
>>> or the system IOMMU is disabled?
>>>
>> Good point.
>> If IOMMU is not enabled, device cannot issue DMA requests with PASID. This
>> API will not be available. Forgot to add dummy functions to the header.
>>
> 
> PASID is a PCI thing, not defined by IOMMU.
> 
> I think the key is physically if IOMMU is disabled, how will root complex
> handle a PCI memory request including a PASID TLP prefix? Does it block
> such request due to no IOMMU to consume PASID or simply ignore PASID
> and continue routing the request to the memory controller?
> 
> If block, then having an iommu interface makes sense.
> 
> If ignore, possibly a DMA API call makes more sense instead, implying that
> this extension can be used even when iommu is disabled.
> 
> I think that is what Baolu wants to point out.

Yes, exactly. Imagining in the VM guest environment, do we require a
vIOMMU for this functionality? vIOMMU is not performance friendly if we
put aside the security considerations.

Best regards,
baolu