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

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

Hi Lu,

On Thu, 9 Dec 2021 10:21:38 +0800, Lu Baolu <baolu.lu@linux.intel.com>
wrote:

> 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.  
> 
Thanks for clarifying, very good point.
Looking at the PCIe spec. I don't see specific rules for RC to ignore or
block PASID TLP if not enabled.
"- A Root Complex that supports PASID TLP Prefixes must have a device
specific mechanism for enabling them. By default usage of PASID TLP
Prefixes is disabled
 - Root Complexes may optionally support TLPs with PASID TLP Prefixes. The
mechanism used to detect whether a Root Complex supports the PASID TLP
Prefix is implementation specific
"
For all practical purposes, why would someone sets up PASID for DMA just to
be ignored? An IOMMU interface makes sense to me.

> 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.
> 
The primary use case for accelerators to use in-kernel DMA will be in
pass-through mode. vIOMMU should be able to do PT with good performance,
right? no nesting, IO page faults.

> Best regards,
> baolu


Thanks,

Jacob
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