Re: [PATCH v3 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices

[Jason Gunthorpe]

On Fri, Mar 06, 2026 at 03:41:15PM -0800, Nicolin Chen wrote:
> Controlled by the IOMMU driver, ATS is usually enabled "on demand" when a
> device requests a translation service from its associated IOMMU HW running
> on the channel of a given PASID. This is working even when a device has no
> translation on its RID (i.e., the RID is IOMMU bypassed).
> 
> However, certain PCIe devices require non-PASID ATS on their RID even when
> the RID is IOMMU bypassed. Call this "always on".
> 
> For instance, the CXL spec notes in "3.2.5.13 Memory Type on CXL.cache":
> "To source requests on CXL.cache, devices need to get the Host Physical
> Address (HPA) from the Host by means of an ATS request on CXL.io."
> 
> In other words, the CXL.cache capability requires ATS; otherwise, it can't
> access host physical memory.
> 
> Introduce a new pci_ats_always_on() helper for the IOMMU driver to scan a
> PCI device and shift ATS policies between "on demand" and "always on".
> 
> Add the support for CXL.cache devices first. Pre-CXL devices will be added
> in quirks.c file.
> 
> Note that pci_ats_always_on() validates against pci_ats_supported(), so we
> ensure that untrusted devices (e.g. external ports) will not be always on.
> This maintains the existing ATS security policy regarding potential side-
> channel attacks via ATS.
> 
> Cc: linux-cxl@xxxxxxxxxxxxxxx
> Suggested-by: Vikram Sethi <vsethi@xxxxxxxxxx>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> ---
>  include/linux/pci-ats.h       |  3 +++
>  include/uapi/linux/pci_regs.h |  1 +
>  drivers/pci/ats.c             | 42 +++++++++++++++++++++++++++++++++++
>  3 files changed, 46 insertions(+)

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason