Re: [RFC PATCH 0/5] x86: dynamic indirect call promotion

[Nadav Amit]

> On Nov 28, 2018, at 5:40 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 
> On Wed, Nov 28, 2018 at 4:38 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> On Wed, Nov 28, 2018 at 07:34:52PM +0000, Nadav Amit wrote:
>>>> On Nov 28, 2018, at 8:08 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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 v1â, ending up not sending itâ
>>> 
>>> There is one issue that I realized while working on the new version: Iâm 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 didnât 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 didnât appear so. Although there are >10000 indirect branches in the
kernel, you donât 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.