Paul A. Clayton (paaronclayton.delete@this.gmail.com) on September 18, 2021 2:35 pm wrote:
> anon (anon.delete@this.ymous.org) on September 15, 2021 12:15 am wrote:
> [snip]
> > Well, I don't think my concern was about permissions at all, but the discussion is nevertheless
> > pertinent. I am not familiar with the PLB though, is there a reference ?
>
> "A Comparison of Protection Lookaside Buffers and the PA-RISC Protection Architecture" (John Wilkes
> and Bart Sears, 1992, PDF) was my exposure to the concept. However, the principle is simply divorcing
> translation from permissions. This is somewhat obvious in that the granularity of permissions need
> not match the granularity of translation (and if one does not have aliases in a virtually tagged cache
> one can defer translation until time to access a memory-side cache or later with the obvious benefit
> of permissions being smaller (and potentially coarser-grained) than translations).
>

With permission coarser-grained than translation how are you going to emulate a Present bit in PTE?
Or do you propose to totally give up on demand paging?

> (There probably is potential for tag compression with temporal sharing of address references of permissions,
> translation, and data caching; a microarchitecture might choose not to fully isolate these aspects.)
>
> (The intent of my four-word no-internal-text post was merely that permission checks
> do not require translation — not that permission checks should necessarily be
> deferred [and I do not need anyone's help to make an ass out of me].)
>
> With respect to handling load-to-store forwarding, if there are no aliases/duplication in a virtually-tagged
> cache, then cache hit status could presumably be used to help manage store-to-load forwarding (in addition
> to not matching the page offset bits). Whether the complexity and area/power costs of managing aliases
> in a virtually-tagged cache are worthwhile for "normal" OSes that allow aliases is a different question
> than whether virtually-tagged caches are technically possible while supporting such OSes. Abnormal OSes
> are also theoretically possible even if the economics would not be favorable.