> On Nov 28, 2018, at 5:40 PM, Andy Lutomirski <l...@kernel.org> wrote: > > On Wed, Nov 28, 2018 at 4:38 PM Josh Poimboeuf <jpoim...@redhat.com> wrote: >> On Wed, Nov 28, 2018 at 07:34:52PM +0000, Nadav Amit wrote: >>>> On Nov 28, 2018, at 8:08 AM, Josh Poimboeuf <jpoim...@redhat.com> wrote: >>>> >>>> On Wed, Oct 17, 2018 at 05:54:15PM -0700, Nadav Amit wrote: >>>>> This RFC introduces indirect call promotion in runtime, which for the >>>>> matter of simplification (and branding) will be called here "relpolines" >>>>> (relative call + trampoline). Relpolines are mainly intended as a way >>>>> of reducing retpoline overheads due to Spectre v2. >>>>> >>>>> Unlike indirect call promotion through profile guided optimization, the >>>>> proposed approach does not require a profiling stage, works well with >>>>> modules whose address is unknown and can adapt to changing workloads. >>>>> >>>>> The main idea is simple: for every indirect call, we inject a piece of >>>>> code with fast- and slow-path calls. The fast path is used if the target >>>>> matches the expected (hot) target. The slow-path uses a retpoline. >>>>> During training, the slow-path is set to call a function that saves the >>>>> call source and target in a hash-table and keep count for call >>>>> frequency. The most common target is then patched into the hot path. >>>>> >>>>> The patching is done on-the-fly by patching the conditional branch >>>>> (opcode and offset) that is used to compare the target to the hot >>>>> target. This allows to direct all cores to the fast-path, while patching >>>>> the slow-path and vice-versa. Patching follows 2 more rules: (1) Only >>>>> patch a single byte when the code might be executed by any core. (2) >>>>> When patching more than one byte, ensure that all cores do not run the >>>>> to-be-patched-code by preventing this code from being preempted, and >>>>> using synchronize_sched() after patching the branch that jumps over this >>>>> code. >>>>> >>>>> Changing all the indirect calls to use relpolines is done using assembly >>>>> macro magic. There are alternative solutions, but this one is >>>>> relatively simple and transparent. There is also logic to retrain the >>>>> software predictor, but the policy it uses may need to be refined. >>>>> >>>>> Eventually the results are not bad (2 VCPU VM, throughput reported): >>>>> >>>>> base relpoline >>>>> ---- --------- >>>>> nginx 22898 25178 (+10%) >>>>> redis-ycsb 24523 25486 (+4%) >>>>> dbench 2144 2103 (+2%) >>>>> >>>>> When retpolines are disabled, and if retraining is off, performance >>>>> benefits are up to 2% (nginx), but are much less impressive. >>>> >>>> Hi Nadav, >>>> >>>> Peter pointed me to these patches during a discussion about retpoline >>>> profiling. Personally, I think this is brilliant. This could help >>>> networking and filesystem intensive workloads a lot. >>> >>> Thanks! I was a bit held-back by the relatively limited number of responses. >> >> It is a rather, erm, ambitious idea, maybe they were speechless :-) >> >>> I finished another version two weeks ago, and every day I think: "should it >>> be RFCv2 or v1”, ending up not sending it… >>> >>> There is one issue that I realized while working on the new version: I’m not >>> sure it is well-defined what an outline retpoline is allowed to do. The >>> indirect branch promotion code can change rflags, which might cause >>> correction issues. In practice, using gcc, it is not a problem. >> >> Callees can clobber flags, so it seems fine to me. > > Just to check I understand your approach right: you made a macro > called "call", and you're therefore causing all instances of "call" to > become magic? This is... terrifying. It's even plausibly worse than > "#define if" :) The scariest bit is that it will impact inline asm as > well. Maybe a gcc plugin would be less alarming?
It is likely to look less alarming. When I looked at the inline retpoline implementation of gcc, it didn’t look much better than what I did - it basically just emits assembly instructions. Anyhow, I look (again) into using gcc-plugins. >>> 1. An indirect branch inside the BP handler might be the one we patch >> >> I _think_ nested INT3s should be doable, because they don't use IST. >> Maybe Andy can clarify. > > int3 should survive recursion these days. Although I admit I'm > currently wondering what happens if one thread puts a kprobe on an > address that another thread tries to text_poke. The issue I regarded is having an indirect call *inside* the the handler. For example, you try to patch the call to bp_int3_handler and then get an int3. They can be annotated to prevent them from being patched. Then again, I need to see how gcc plugins can get these annotations. > > Also, this relpoline magic is likely to start patching text at runtime > on a semi-regular basis. This type of patching is *slow*. Is it a > problem? It didn’t appear so. Although there are >10000 indirect branches in the kernel, you don’t patch too many of them even you are doing relearning. > >>> 2. An indirect branch inside an interrupt or NMI handler might be the >>> one we patch >> >> But INT3s just use the existing stack, and NMIs support nesting, so I'm >> thinking that should also be doable. Andy? > > In principle, as long as the code isn't NOKPROBE_SYMBOL-ified, we > should be fine, right? I'd be a little nervous if we get an int3 in > the C code that handles the early part of an NMI from user mode. It's > *probably* okay, but one of the alarming issues is that the int3 > return path will implicitly unmask NMI, which isn't fantastic. Maybe > we finally need to dust off my old "return using RET" code to get rid > of that problem. So it may be possible. It would require having a new text_poke_bp() variant for multiple instructions. text_poke_bp() might be slower though.
