On 8/10/23 19:12, Dexuan Cui wrote: > GHCI spec for TDX 1.0 says that the MapGPA call may fail with the R10 > error code = TDG.VP.VMCALL_RETRY (1), and the guest must retry this > operation for the pages in the region starting at the GPA specified > in R11. > > When a fully enlightened TDX guest runs on Hyper-V, Hyper-V can return > the retry error when set_memory_decrypted() is called to decrypt up to > 1GB of swiotlb bounce buffers. This changelog is not great. It gives zero background and wastes bytes on telling me which register the error code is in (I don't care in a changelog) and then using marketing fluff words like "fully enlightened". Let's stick to the facts, give some background, and also avoid regurgitating the GHCI, eh? How's this? x86/tdx: Retry partially-completed page conversion hypercalls TDX guest memory is private by default and the VMM may not access it. However, in cases where the guest needs to share data with the VMM, the guest and the VMM can coordinate to make memory shared between them. The guest side of this protocol includes the "MapGPA" hypercall. This call takes a guest physical address range. The hypercall spec (aka. the GHCI) says that the MapGPA call is allowed to return partial progress in mapping this range and indicate that fact with a special error code. A guest that sees such partial progress is expected to retry the operation for the portion of the address range that was not completed. Hyper-V does this partial completion dance when set_memory_decrypted() is called to "decrypt" swiotlb bounce buffers that can be up to 1GB in size. It is evidently the only VMM that does this, which is why nobody noticed this until now.
