Re: IOAT DMA w/IOMMU

[Logan Gunthorpe <logang@deltatee.com>]

On 10/08/18 06:53 PM, Kit Chow wrote:
> I was able to finally succeed in doing the dma transfers over ioat only
> when prot has DMA_PTE_WRITE set by setting the direction to either
> DMA_FROM_DEVICE or DMA_BIDIRECTIONAL. Any ideas if the prot settings
> need to be changed? Are there any bad side effects if I used
> DMA_BIDIRECTIONAL?

Good to hear it. Without digging into the direction much all I can say
is that it can sometimes be very confusing what the direction is. Adding
another PCI device just adds to the confusion.

I believe, the direction should be from the IOAT's point of view. So if
the IOAT is writing to the BAR you'd set DMA_FROM_DEVICE (ie. data is
coming from the IOAT) and if it's reading you'd set DMA_TO_DEVICE (ie.
data is going to the IOAT).

Using DMA_BIDIRECTIONAL just forgoes any hardware security / protection
that the buffer would have in terms of direction. Generally it's good
practice to use the strictest direction you can.

> Given that using the pci bar address as is without getting an iommu
> address results in the same "PTE Write access" error, I wonder if there
> is some internal 'prot' associated with the non-translated pci bar
> address that just needs to be tweaked to include DMA_PTE_WRITE???

No, I don't think so. The 'prot' will be a property of the IOMMU. Not
having an entry is probably just the same (from the perspective of the
error you see) as only having an entry for reading.

Logan