Hi,
On 10/23/18 12:03 AM, Jean-Philippe Brucker wrote:
#1: Given that PASID is a system wide resource, during boot iommu driver needs
to scan and set the max PASID to be no more than min(iommu_supported_max_pasid)
of all available IOMMU's.
#2: In addition if any device supports less than the choosen system max PASID
we should simply disable PASID on that device.
The reason is if the process were to bind to 2 or more accelerators and
the second device has a limit smaller than the first that the application
already attached, the second attemt to bind would fail. (Because
we would use the same PASID for a single process)
But that's not reason enough to completely disable PASID for the device,
it could be the only one bound to that process, or PASID could be only
used privately by the host device driver.
In addition for Intel IOMMU's PASID is a system wide resource. We have
a virtual capability for vIOMMU's to allocate PASID's. At the time of
allocation we don't know what device this PASID is even being allocated
for.
Ah, I had missed that part, I thought the PV allocation had the device
ID as well. That's a significant change.
I was still hoping that we could go back to per-device PASID spaces in
the host, since I still haven't seen any convincing argument in favor of
system-wide PASID. Get rid of #1 in order to solve #2, and as a bonus
support more PASIDs in total. Until now PASID was a per-device resource
except for choices made when writing the IOMMU driver.
System wide PASID makes things simple and workable in many use cases
although it looks a bit coarse grained.
For an example, in a use case of one application manipulating multiple
devices with a single PASID, it will be complicated to find a PASID
suitable for all the per-device PASID spaces. It will become more
complicated when it comes to PV allocation since PV context might have
no device information when it requires a PASID.
Best regards,
Lu Baolu
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