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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). > > 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.. I raised my concerns about CXL.cache and virtualization at LPC: https://lpc.events/event/19/contributions/2173/attachments/1842/3940/LPC_2025_CXL_CACHE.pdf I expose there some concerns, although I admit some could be due to my twisted understanding of what CXL specs states, but regarding IOMMU/ATS, my view is ATS is not safe enough ... what I guess is a matter of opinion (trusted device based on vendor confirming it is an "official" device not enough for paranoid mode with vendors subjected to governments "agencies" actions/pressures). But this links to what I think Jason points out about ATS security features where the IOMMU hardware can be configured for checking those translated PCIe accesses as well, if the host owner/admin paranoid mind decides so. With CXL cache that is not possible since the route is through a different link and AFAIK, there is no support for something like this by current implementations. I think it could be implemented without impacting the gains from CXL.cache, but that is another story. So, FWIW, I think it should not be enabled by default. Thank you, Alejandro > Jason >