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

Jason Gunthorpe
2021-09-29T12:36:42Z

On Wed, Sep 29, 2021 at 08:48:28AM +0000, Tian, Kevin wrote:

> ARM:
> - set to snoop format if IOMMU_CACHE
> - set to nonsnoop format if !IOMMU_CACHE
> (in both cases TLP snoop bit is ignored?)

Where do you see this? I couldn't even find this functionality in the
ARM HW manual??

What I saw is ARM linking the IOMMU_CACHE to a IO PTE bit that causes
the cache coherence to be disabled, which is not ignoring no snoop.

> I didn't identify the exact commit for above meaning change.
>
> Robin, could you help share some thoughts here?

It is this:

static int dma_info_to_prot(enum dma_data_direction dir, bool coherent,
unsigned long attrs)
{
int prot = coherent ? IOMMU_CACHE : 0;

Which sets IOMMU_CACHE based on:

static void *iommu_dma_alloc(struct device *dev, size_t size,
dma_addr_t *handle, gfp_t gfp, unsigned long attrs)
{
bool coherent = dev_is_dma_coherent(dev);
int ioprot = dma_info_to_prot(DMA_BIDIRECTIONAL, coherent, attrs);

Driving IOMMU_CACHE from dev_is_dma_coherent() has *NOTHING* to do
with no-snoop TLPs and everything to do with the arch cache
maintenance API

Jason