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GPU DMA issue diagnoses and impact (missing DMA kernel support beyond 32 bits)

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JeremyRand:
A few questions about this:


* Does the Linux 5.4 compatibility mode fix all affected PCIe devices, or only AMD GPU's?  If the latter, does it apply to the Radeon driver, the AMDGPU driver, or both?
* If I'm on Linux 5.4 or later, how would I test whether the compatibility mode is in use for a given PCIe device?
* Am I correct in understanding that the compatibility mode is Linux-specific, and thus would need to be re-implemented in other kernels to fix the broken hardware in those kernels?
* Does the compatibility mode make memory allocation behavior for affected PCIe devices more predictable, even slightly?  (E.g. does it artificially restrict the PCIe device's memory space, as seen by the host, to a 40-bit window, rather than the much larger 64-bit window that would otherwise exist?)  If so, doesn't this decrease security (e.g. by making ASLR less effective)?  (I'm not asking if this security degradation is significant to average users, just whether it exists at all.)

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