Re: [RFC net-next 1/4] net: Allow non parent devices to be used for ZC DMA

On Fri, 11 Jul 2025 02:52:23 +0000 Parav Pandit wrote:

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On Thu, 3 Jul 2025 11:58:50 +0000 Parav Pandit wrote:  

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In my head subfunctions are a way of configuring a PCIe PASID ergo
they _only_ make sense in context of DMA.  

SF DMA is on the parent PCI device.

SIOV_R2 will have its own PCI RID which is ratified or getting ratified.
When its done, SF (as SIOV_R2 device) instantiation can be extended
with its own PCI RID. At that point they can be mapped to a VM.  

AFAIU every PCIe transaction for a queue with a PASID assigned should have a
PASID prefix. Why is a different RID necessary?
CPUs can't select IOMMU context based on RID+PASID?  

It can, however,
PASID is meant to be used for process isolation and not expected to
be abused for identify the device. Doing so, would also prohibits
using PASID inside the VM. It requires another complex vPASID to
pPASID translation.

Tagging MSI-X interrupts with PASID is another challenge.
For CC defining isolation boundary with RID+PASID was yet another
hack.

There were other issues in splitting PASID for device scaling vs
process scaling for dual use.

So it was concluded to opt to avoid that abuse and use the standard
RID construct for device identification.

I see, that explains it. Thanks Parav!