Re: [PATCH RFCv1 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices

Jason Gunthorpe
2026-01-22T13:15:24Z

On Thu, Jan 22, 2026 at 09:17:27AM +0800, Baolu Lu wrote:
> On 1/21/26 21:03, Jason Gunthorpe wrote:
> > On Wed, Jan 21, 2026 at 10:03:07AM +0000, Jonathan Cameron wrote:
> > > On Wed, 21 Jan 2026 08:01:36 +0000
> > > "Tian, Kevin"<kevin.tian@intel.com> wrote:
> > >
> > > > +Dan. I recalled an offline discussion in which he raised concern on
> > > > having the kernel blindly enable ATS for cxl.cache device instead of
> > > > creating a knob for admin to configure from userspace (in case
> > > > security is viewed more important than functionality, upon allowing
> > > > DMA to read data out of CPU caches)...
> > > >
> > > +CC Linux-cxl
> > A cxl.cache device supporting ATS will automatically enable ATS today
> > if the kernel option to enable translation is set.
> >
> > Even if the device is marked untrusted by the PCI layer (eg an
> > external port).
>
> I don't follow here. The untrusted check is now in pci_ats_supported():
>
> /**
> * pci_ats_supported - check if the device can use ATS
> * @dev: the PCI device
> *
> * Returns true if the device supports ATS and is allowed to use it, false
> * otherwise.
> */
> bool pci_ats_supported(struct pci_dev *dev)
> {
> if (!dev->ats_cap)
> return false;
>
> return (dev->untrusted == 0);
> }
> EXPORT_SYMBOL_GPL(pci_ats_supported);
>
> The iommu drivers (intel/amd/arm-smmuv3) all call pci_ats_supported()
> before enabling ATS on a device. Anything I missed?

No, not at all, I forgot about this!

Jason