What is the instruction tlbiel and what does it do?

Question

I've recently come across an instruction called tlbiel and I'm curious as to what it means and what purpose it serves. After doing some intense Google searching, I saw one IBM document that lead me to believe it's hypervisor-related. The current instruction that I'm facing is tlbiel r3, 1 and after studying the opcode (0x7C201A24), I've come to the conclusion that it roughly translates to mtspr 3, 1; the only difference being that the expected value of 467 at 21 - 30 in mtspr's opcode has changed to 274. However, before tlbiel gets executed, instructions li r3, 0x3FF & sldi r3, r3, 32 are introduced directly before, causing r3 to contain 0x00003FF000000000.

As far as I know, there's no SPR with the value of 3 and there's no real documentation on tlbiel at all (that I can find). What does this instruction accomplish?

Answer 1

tlbiel is "TLB Invalidate Entry Local": it invalidates an entry in the TLB [Translation Look-aside Buffer]. As comments on the question surmised, it is related to a regular TLB invalidate, but it is "local" in that it only invalidates TLB entries on the thread executing the tlbiel instruction.

For more details, the usual source is the Power ISA. It explains the instruction in very fine detail, including what it does with the register that you specify.

There are a few versions of the ISA floating around. The two that are easiest to find are:

• version 2.07, which deals with POWER8: https://openpowerfoundation.org/?resource_lib=ibm-power-isa-version-2-07-b
• version 3, which deals with the forthcoming POWER9: https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0

You will need to sign up for an IBM ID to download the files, but they are free. If you're doing assembler level development on a PowerPC chip they are absolutely invaluable resources.

A couple of final things:

• You've mentioned that you're running on an Xbox 360. That supports a slightly different instruction set. If you can find the ISA corresponding to that specific processor, so much the better: while most things stay the same, some of the details do change. (I don't have any experience with anything older than a Power8 - so apologies that I can't be more specific here!)
• The Linux kernel is by far the best open-source documentation of how to do low-level things on Power processors. There's code in there that deals with TLB invalidation, and in particular when tlbiel is sufficient and when a non-local version is required.