Oh, this will also absorb multilevel TLBs too, like how ARM has second level translation in some cases. This isn't really implemented in x86, but could also be used for it's multilevel translation in SVM and VT's nested page table schemes.
Gabe On Sat, Sep 19, 2020 at 8:25 PM Gabe Black <[email protected]> wrote: > Hi folks. I've been thinking about how to rework the > scanning-through-page-translation thing we currently do when translating a > region of addresses through both the ITB and DTB. We currently do that one > page at a time by trying one, and then the other. That requires knowing > what "the" page size is, which introduces a dependence on the ISA and also > constraints things to a single page size. > > One improvement that I think makes sense is to instead use an approach > where you'd ask for a translation for a region and let whatever is > translating for you decide how to break things up. Then it can use a single > page size, the size of whatever the mapping is, a single byte, etc., as it > sees fit without there ever needing to be a particular page size. This > would fit mostly nicely with the idea of a range based for loop or > generator object. > > One problem with this approach is how we try one TLB and then the other if > we can't get a translation. With a range based for loop, there isn't a good > way that I'm aware of to iterate over two different objects at the same > time, and also there wouldn't be any coordination between the TLBs. For > instance, what should happen in both have a translation? Or if they're to > the same place, who's idea of the size of the step takes precedence? > > That led me to the idea of merging the TLBs into a single object called > the MMU. This fits pretty well with the structure of actual hardware, and > would also absorb the "UnifiedTLB" concept which was added to the CPU, > since the structure of the TLBs would no longer be visible to the CPU. > > I think this makes a lot of sense and will probably take a stab at it this > weekend, but since it's a structural shift that will be fairly visible in a > lot of places I wanted to let everybody know a little ahead of time in case > there are any big concerns. > > Gabe >
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