Re: [PATCH v4 1/5] PCI/P2PDMA: Don't enforce ACS check for device functions of Intel GPUs

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Sep 22, 2025 at 06:59:10AM +0000, Kasireddy, Vivek wrote:

> > You can't translate a dma_addr_t to anything in the Xe PF driver
> > anyhow, once it goes through the IOMMU the necessary information is lost.
> Well, I already tested this path (via IOMMU, with your earlier vfio-pci +
> dmabuf patch that used dma_map_resource() and also with Leon's latest
> version) and found that I could still do the translation in the Xe PF driver
> after first calling iommu_iova_to_phys().

I would NAK any driver doing something so hacky. You are approaching
this completely wrong to abuse things like this.

> that there are no memory copies and the CPU is not involved. And, I don't see
> any other way to do this because I don't believe the exporter can provide a
> DMA address that the importer can use directly without any translation, which
> seems unavoidable in this case.

This is sadly how dmabuf is designed, please talk to Christain to
figure out how to exchage the right kind of address.

> use-case. And, I sincerely hope that it will work, otherwise I don't see
> any viable path forward for what I am trying to do other than using this
> quirk and translation. 

You will not get a quirk.

> PCIe fabric). Isn't that one of the main ideas behind using quirks -- to
> address hardware limitations?

This is not a hardware limitation. You are using DMABUF wrong, writing
hacks into the PF driver and then trying to make up a fake ACS quirk
:(

Jason