Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

Jason Gunthorpe via iommu
2021-09-29T23:43:09Z

On Wed, Sep 29, 2021 at 03:57:20PM -0700, Jacob Pan wrote:
> Hi Jason,
>
> On Wed, 29 Sep 2021 16:39:53 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> > On Wed, Sep 29, 2021 at 12:37:19PM -0700, Jacob Pan wrote:
> >
> > > For #2, it seems we can store the kernel PASID in struct device. This
> > > will preserve the DMA API interface while making it PASID capable.
> > > Essentially, each PASID capable device would have two special global
> > > PASIDs:
> > > - PASID 0 for DMA request w/o PASID, aka RID2PASID
> > > - PASID 1 (randomly selected) for in-kernel DMA request w/
> > > PASID
> >
> > This seems reasonable, I had the same thought. Basically just have the
> > driver issue some trivial call:
> > pci_enable_pasid_dma(pdev, &pasid)
> That would work, but I guess it needs to be an iommu_ call instead of pci_?

Which ever makes sense.. The API should take in a struct pci_device
and return a PCI PASID - at least as a wrapper around a more generic
immu api.

> I think your suggestion is more precise, in case the driver does not want
> to do DMA w/ PASID, we can do less IOTLB flush (PASID 0 only).

Since it is odd, and it may create overhead, I would do it only when
asked to do it

> > Having multiple RID's pointing at the same IO page table is something
> > we expect iommufd to require so the whole thing should ideally fall
> > out naturally.

> That would be the equivalent of attaching multiple devices to the same
> IOMMU domain. right?

Effectively..

Jason