Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

[Peter Zijlstra]

On Tue, Jan 08, 2019 at 09:47:18AM +0200, Adrian Hunter wrote:
> On 7/01/19 6:32 PM, Peter Zijlstra wrote:
> > On Thu, Jan 03, 2019 at 02:18:15PM -0800, Andi Kleen wrote:
> >> Nadav Amit <namit@xxxxxxxxxx> writes:
> >>>
> >>> - Do we use periodic learning or not? Josh suggested to reconfigure the
> >>>   branches whenever a new target is found. However, I do not know at
> >>>   this time how to do learning efficiently, without making learning much
> >>>   more expensive.
> >>
> >> FWIW frequent patching will likely completely break perf Processor Trace
> >> decoding, which needs a somewhat stable kernel text image to decode the
> >> traces generated by the CPU. Right now it relies on kcore dumped after
> >> the trace usually being stable because jumplabel changes happen only
> >> infrequently. But if you start patching frequently this assumption will
> >> break.
> >>
> >> You would either need a way to turn this off, or provide
> >> updates for every change to the trace, so that the decoder can
> >> keep track.
> > 
> > I'm thining it would be entirely possible to create and feed text_poke
> > events into the regular (!aux) buffer which can be timestamp correlated
> > to the PT data.
> 
> To rebuild kernel text from such events would require a starting point.
> What is the starting point?  The problem with kcore is that people can
> deconfig it without realising it is needed to enable the tracing of kernel
> self-modifying code.  It would be nice if it was all tied together, so that
> if someone selects the ability to trace kernel self-modifying code, then all
> the bits needed are also selected.  Perhaps we should expose another ELF
> image that contains only kernel executable code, and take the opportunity to
> put the symbols in it also.

Meh; you always need a magic combo of CONFIG symbols to make stuff work.
We don't even have a CONFIG symbol for PT, so if you really care you
should probably start there.

If you want symbols; what stops us from exposing kallsyms in kcore as
is?

> Also what about BPF jitted code?  Will it always fit in an event?  I was
> thinking of trying to add a way to prevent temporarily the unload of modules
> or jitted code, which would be a good-enough solution for now.

We're working on BPF and kallsym events, those should, esp. when
combined with kcore, allow you to extract the actual instructions.

We don't have module events, but I suppose the kallsym events should
cover module loading (a new module results in lots of new symbols after
all).

With all that there is still a race in that nothing blocks module-unload
while we're still harvesting the information, not sure how/if we want to
cure that.