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

[Peter Zijlstra]

On Tue, Jan 08, 2019 at 12:47:42PM -0800, Nadav Amit wrote:

> A general solution is more complicated, however, due to the racy nature of
> cross-modifying code. There would need to be TSC recording of the time
> before the modifications start and after they are done.
> 
> BTW: I am not sure that static-keys are much better. Their change also
> affects the control flow, and they do affect the control flow.

Any text_poke() user is a problem; which is why I suggested a
PERF_RECORD_TEXT_POKE that emits the new instruction. Such records are
timestamped and can be correlated to the trace.

As to the racy nature of text_poke, yes, this is a wee bit tricky and
might need some care. I _think_ we can make it work, but I'm not 100%
sure on exactly how PT works, but something like:

 - write INT3 byte
 - IPI-SYNC

and ensure the poke_handler preserves the existing control flow (which
it currently does not, but should be possible).

 - emit RECORD_TEXT_POKE with the new instruction

at this point the actual control flow will be through the INT3 and
handler and not hit the actual instruction, so the actual state is
irrelevant.

 - write instruction tail
 - IPI-SYNC
 - write first byte
 - IPI-SYNC

And at this point we start using the new instruction, but this is after
the timestamp from the RECORD_TEXT_POKE event and decoding should work
just fine.