Re: [PATCH 0/5] perf: add support for capturing skid IP

[Stephane Eranian]

Andi,

On Thu, Jun 15, 2017 at 4:18 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>> Looking at this approach, the user interface is straightforward,
>> implementation in the x86 code is a bit more hairy because of the way
>> the branch_stack is captured, via the cpuc->lbr_entries. If you assume
>> that SKID_IP cannot be used with any other branch stack mode, then it
>> is easy. It becomes messy if you don't.
>
> That should be fine. After all if you have real LBRs you don't need
> the skid IP.
>
Yes, you still do. This is not the same thing. LBR captures only taken branches.
I care about taken AND non-taken branches and I don't want to sample on a
non-taken event, assuming it is available.

You need to inject the skid ip into the LBR stack somehow. Either
directly in the
registers or in the cpuc->lbr_entries. The injection can only happen
on interrupt
whereas the LBR captures 32 branches. So you'd have the skid ip info
only for the
most recent branch in the stack.