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

[Nadav Amit]

> On Jan 8, 2019, at 11:01 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Tue, Jan 08, 2019 at 10:28:02AM -0800, Nadav Amit wrote:
>> Is it really that important for debugging to get the instructions at the
>> time of execution? Wouldnât it be easier to annotate the instructions that
>> might change? After all, it is not as if any instruction can change to any
>> other instruction.
> 
> I think PT has a bitstream encoding of branch-taken; to decode and
> follow the actual code-flow you then need to have the actual and
> accurate branch target from the code. If we go muck about with the code
> and change that, decoding gets somewhat 'tricky'.
> 
> Or something along those lines..

Thanks for the explanation (I now see it in the SDM and sources).

Basically, the updates should not be done too frequently, and I can expose
an interface to suspend them (this will not affect correctness). I think
this can be the easiest solution, which should not affect the workload
execution too much.

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.