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

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Message-ID: <fa3e55e1-3cf4-4b73-8b5e-192a19a18f3b@linux.intel.com>
Date: Thu, 22 Jan 2026 09:17:27 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>,
 Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, Nicolin Chen <nicolinc@...dia.com>,
 "will@...nel.org" <will@...nel.org>,
 "robin.murphy@....com" <robin.murphy@....com>,
 "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
 "Williams, Dan J" <dan.j.williams@...el.com>,
 "joro@...tes.org" <joro@...tes.org>, "praan@...gle.com" <praan@...gle.com>,
 "miko.lenczewski@....com" <miko.lenczewski@....com>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
 linux-cxl@...r.kernel.org
Subject: Re: [PATCH RFCv1 1/3] PCI: Allow ATS to be always on for CXL.cache
 capable devices

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@...el.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?

> 
> Yes this is effectively a security issue, but it is not really a CXL
> specific problem.
> 
> We might perfer to not enable ATS for untrusted devices and then fail to
> load drivers for "ats always on" cases.
> 
> Or maybe we can enable one of the ATS security features someday,
> though I wonder if those work for CXL..

Thanks,
baolu