Messages in this thread | | | Date | Mon, 8 May 2023 09:40:14 -0700 | From | Jacob Pan <> | Subject | Re: [PATCH] iommu: Add Kconfig help text for IOMMU_SVA |
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Hi Linus,
On Sun, 7 May 2023 11:52:18 -0700, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, May 6, 2023 at 3:03 PM Jacob Pan <jacob.jun.pan@linux.intel.com> > wrote: > > > > Right, how about IOMMU_SHARING_CPU_PGTABLE? > > I think from a VM / process angle, I'd actually prefer calling the > "pasid" part just that: IOMMU_PASID. > > The VM code certainly understands about address space IDs, even if > people have called them different things: normal people called them > ASID's long ago, then Intel at some pointed decided that "PCID" made > sense as a name (narrator: "no it didn't"), and then you got that > combined "PASID" thing. > > Now, it may be that this then goes hand-in-hand with other IOMMU code > that isn't *about* PASID itself, but that depends on PASID's being > present, and so I'd just expect IOMMU_PASID to be one of those options > that are selected by other options. > > So maybe there is some part of IOMMU_SVA that is not about PASID > itself, but I really think that the PASID code itself should just have > that CONFIG_PASID around it. Conversely, we could also have some part of PASID that is not about SVA. e.g. Today, on PASID enabled IOMMUs, DMA request w/o PASID (legacy) uses a special PASID 0. This has nothing to do with mm->pasid.
> End result: from a legibility standpoint, I think it could be as > simple as having that > > config IOMMU_SVA > > option have a "select IOMMU_PASID". > Then make the VM/process PASID code depend on that. Maybe the "struct > device *" stuff makes more sense under CONFIG_IOMMU_SVA, ie things > like iopf_queue_add_device() and friends. right, we don;t support non-PASID IOPF.
> How does that sound? Maybe those two options then always end up going > together, but even if that is the case, I think from a VM/process > standpoint it makes a lot more sense to simply have a "PASID enabled" > option. It's much more understandable in that context, while something > like "IOMMU_SVA" really is just a random jumble of letters to a VM > person. My only concern is the case above where DMA API uses a PASID for legacy DMA requests w/o PASID. I am also trying to add non-zero PASID for Intel's ENQCMDS. https://lore.kernel.org/linux-iommu/20230427174937.471668-1-jacob.jun.pan@linux.intel.com/ The PASIDs used in this case uses IOVA page tables, not shared with any mm_struct.
From this use case, we need to select IOMMU_PASID, but not necessarily for mm->pasid which IMHO is only meaningful when IOMMU shares page tables with the CPU.
> And while the individual words in IOMMU_SHARING_CPU_PGTABLE all make > sense, it's not clear what the combination means, and why it should > have anything to do with then having an address space identifier for > it. > > Linus >
Thanks,
Jacob
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