Why not have fixed sized MMT tables at all levels ?

riscv / riscv-smmtt

This specification will define the RISC-V privilege ISA extensions required to support Supervisor Domain isolation for multi-tenant security use cases e.g. confidential-computing, trusted platform services, fault isolation and so on.

https://jira.riscv.org/browse/RVG-65

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Why not have fixed sized MMT tables at all levels ? #62

Closed avpatel closed 1 month ago

avpatel commented 2 months ago

The MTT tables at different levels are not of same size and MMT L2 is particularly very large. This makes memory management for MTT tables more involved. Instead of this, if all MTT tables were of same size (just like MMU or IOMMU page tables) then it will be very easy to manage a pool of fixed size MMT tables.

rsahita commented 2 months ago

this was discussed very early on in the TG - the MTT structures are biased towards minimizing lookups, and being compact. Also important is to support all arch page sizes. Another key SW aspect is top level L3, L2 MTT structures will need to be initialized early on in system initialization by the RDSM - lower level structures L1 are 4KiB based so as differing permissions are required, large pages can be fragmented easily and can L1 table pages can be flexibly allocated from pool of 4KiB pages.

rsahita commented 1 month ago

@avpatel I added a check for pa width in the MTT walk - ptal at PR #88 This should address your concern with overcommitting space for L3 and L2 tables. Separately I will also send a PR for adding a type field in L3 which will allow for further flexibility.

avpatel commented 1 month ago

Thanks, this looks reasonable for platforms with smaller system address space.

rsahita commented 1 month ago

PR #88 addresses this issue so closing this one.