RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

Tian, Kevin
2021-09-30T08:49:15Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, September 23, 2021 8:22 PM
>
> > > These are different things and need different bits. Since the ARM path
> > > has a lot more code supporting it, I'd suggest Intel should change
> > > their code to use IOMMU_BLOCK_NO_SNOOP and abandon
> IOMMU_CACHE.
> >
> > I didn't fully get this point. The end result is same, i.e. making the DMA
> > cache-coherent when IOMMU_CACHE is set. Or if you help define the
> > behavior of IOMMU_CACHE, what will you define now?
>
> It is clearly specifying how the kernel API works:
>
> !IOMMU_CACHE
> must call arch cache flushers
> IOMMU_CACHE -
> do not call arch cache flushers
> IOMMU_CACHE|IOMMU_BLOCK_NO_SNOOP -
> dot not arch cache flushers, and ignore the no snoop bit.

Who will set IOMMU_BLOCK_NO_SNOOP? I feel this is arch specific
knowledge about how cache coherency is implemented, i.e.
when IOMMU_CACHE is set intel-iommu driver just maps it to
blocking no-snoop. It's not necessarily to be an attribute in
the same level as IOMMU_CACHE?

>
> On Intel it should refuse to create a !IOMMU_CACHE since the HW can't
> do that.

Agree. In reality I guess this is not hit because all devices are marked
coherent on Intel platforms...

Baolu, any insight here?

Thanks
Kevin