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riscv-non-isa / riscv-iommu

RISC-V IOMMU Specification
https://jira.riscv.org/browse/RVG-55
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Question about the description of t2gpa usage scenario #311

Closed viktoryou closed 2 months ago

viktoryou commented 2 months ago

From my understanding, for a PCIe ATS Translation Request, the usage scenario of t2gpa when enabled would typically be like:

It seems like t2gpa means translating to gpa. Is the pattern above a fixed step-by-step usage flow? In other words, could we say it is expected a untranslated request would be immediately followed by a related translated request(with same gpa)? If this is the most common case, then I guess the translation cache is important for the second stage when t2gpa is enabled.

ved-rivos commented 2 months ago

firstly, untranslated request from device triggers IOMMU to perform the two-stage address translation, and GPA returns and gets cached in device ATC,

Its only applicable to ATS Translation requests - not untranslated request. On a ATS Translation request, the IOMMU does an IOVA -> GPA -> SPA translation. However, it only returns the GPA and not the SPA in the completion to the device. The translation from GPA->SPA is done to ensure there are no faults and to determine accurate permissions.

A translation request in this usage would be followed by a translated requested using the translated address - a GPA in this configuration. The GPA in the translated request is then again translated to an SPA and the permissions verified.

viktoryou commented 2 months ago

That's clear. Thanks a lot.