VIPT Cache: Connection between TLB & Cache?

问题

I just want to clarify the concept and could find detail enough answers which can throw some light upon how everything actually works out in the hardware. Please provide any relevant details.

In case of VIPT caches, the memory request is sent in parallel to both the TLB and the Cache.

From the TLB we get the traslated physical address. From the cache indexing we get a list of tags (e.g. from all the cache lines belonging to a set).

Then the translated TLB address is matched with the list of tags to find a candidate.

• My question is where is this check performed ?
  • In Cache ?
  • If not in Cache, where else ?
• If the check is performed in Cache, then
  • is there a side-band connection from TLB to the Cache module to get the translated physical address needed for comparison with the tag addresses?

Can somebody please throw some light on "actually" how this is generally implemented and the connection between Cache module & the TLB(MMU) module ?

I know this dependents on the specific architecture and implementation. But, what is the implementation which you know when there is VIPT cache ?

Thanks.

回答1:

At this level of detail, you have to break "the cache" and "the TLB" down into their component parts. They're very tightly interconnected in a design that uses the VIPT speed hack of translating in parallel with tag fetch (i.e. taking advantage of the index bits all being below the page offset and thus being translated "for free". Related: Why is the size of L1 cache smaller than that of the L2 cache in most of the processors?)

The L1dTLB itself is a small/fast Content addressable memory with (for example) 64 entries and 4-way set associative (Intel Skylake). Hugepages are often handled with a second (and 3rd) array checked in parallel, e.g. 32-entry 4-way for 2M pages, and for 1G pages: 4-entry fully (4-way) associative.

But for now, simplify your mental model and forget about hugepages. The L1dTLB is a single CAM, and checking it is a single lookup operation.

"The cache" consists of at least these parts:

• the SRAM array that stores the tags + data in sets
• control logic to fetch a set of data+tags based on the index bits. (High-performance L1d caches typically fetch data for all ways of the set in parallel with tags, to reduce hit latency vs. waiting until the right tag is selected like you would with larger more highly associative caches.)
• comparators to check the tags against a translated address, and select the right data if one of them matches, or trigger miss-handling. (And on hit, update the LRU bits to mark this way as Most Recently Used)

The L1dTLB is not really separate from the L1D cache. I don't actually design hardware, but I think a load execution unit in a modern high-performance design works something like this:

• AGU generates an address from register(s) + offset.

  (Fun fact: Sandybridge-family optimistically shortcuts this process for simple addressing mode: [reg + 0-2047] has 1c lower load-use latency than other addressing modes, if the reg value is in the same 4k page as reg+disp. Is there a penalty when base+offset is in a different page than the base?)

• The index bits come from the offset-within-page part of the address, so they don't need translating from virtual to physical. Or translation is a no-op. This VIPT speed with the non-aliasing of a PIPT cache works as long as L1_size / associativity <= page_size. e.g. 32kiB / 8-way = 4k pages.

  The index bits select a set. Tags+data are fetched in parallel for all ways of that set. (This costs power to save latency, and is probably only worth it for L1. Higher-associativity (more ways per set) L3 caches definitely not)

• The high bits of the address are looked up in the L1dTLB CAM array.
• The tag comparator receives the translated physical-address tag and the fetched tags from that set.

• If there's a tag match, the cache extracts the right bytes from the data for the way that matched (using the offset-within-line low bits of the address, and the operand-size).

  Or instead of fetching the full 64-byte line, it could have used the offset bits earlier to fetch just one (aligned) word from each way. CPUs without efficient unaligned loads are certainly designed this way. I don't know if this is worth doing to save power for simple aligned loads on a CPU which supports unaligned loads.

  But modern Intel CPUs (P6 and later) have no penalty for unaligned load uops, even for 32-byte vectors, as long as they don't cross a cache-line boundary. Byte-granularity indexing for 8 ways in parallel probably costs more than just fetching the whole 8 x 64 bytes and setting up the muxing of the output while the fetch+TLB is happening, based on offset-within-line, operand-size, and special attributes like zero- or sign-extension, or broadcast-load. So once the tag-compare is done, the 64 bytes of data from the selected way might just go into an already-configured mux network that grabs the right bytes and broadcasts or sign-extends.

  AVX512 CPUs can even do 64-byte full-line loads.

---

If there's no match in the L1dTLB CAM, the whole cache fetch operation can't continue. I'm not sure if / how CPUs manage to pipeline this so other loads can keep executing while the TLB-miss is resolved. That process involves checking the L2TLB (Skylake: unified 1536 entry 12-way for 4k and 2M, 16-entry for 1G), and if that fails then with a page-walk.

I assume that a TLB miss results in the tag+data fetch being thrown away. They'll be re-fetched once the needed translation is found. There's nowhere to keep them while other loads are running.

At the simplest, it could just re-run the whole operation (including fetching the translation from L1dTLB) when the translation is ready, but it could lower the latency for L2TLB hits by short-cutting the process and using the translation directly instead of putting it into L1dTLB and getting it back out again.

Obviously that requires that the dTLB and L1D are really designed together and tightly integrated. Since they only need to talk to each other, this makes sense. Hardware page walks fetch data through the L1D cache. (Page tables always have known physical addresses to avoid a catch 22 / chicken-egg problem).

is there a side-band connection from TLB to the Cache?

I wouldn't call it a side-band connection. The L1D cache is the only thing that uses the L1dTLB. Similarly, L1iTLB is used only by the L1I cache.

If there's a 2nd-level TLB, it's usually unified, so both the L1iTLB and L1dTLB check it if they miss. Just like split L1I and L1D caches usually check a unified L2 cache if they miss.

Outer caches (L2, L3) are pretty universally PIPT. Translation happens during the L1 check, so physical addresses can be sent to other caches.

来源：https://stackoverflow.com/questions/46480015/vipt-cache-connection-between-tlb-cache