Re: [PATCH 0/2] target/ppc: Fix PMU instruction counting

[Richard Henderson <richard.henderson@linaro.org>]

On 5/21/24 21:04, Nicholas Piggin wrote:
> The crux of the problem being that dynamic exits from a TB would
> not count instructions previously executed in the TB. I don't
> know how important it is for PMU to count instructions exactly,
> however for instruction replay this can lead to different counts
> for the same execution (e.g., because TBs can be different sized)
> and that blows up reverse debugging.
> 
> I posted something on this out before, but missed a few things
> (most notably faulting memory access). And found that forcing 1
> insn per TB seems to be the only feasible way to do this.
> 
> Sorry to ping you on this again Richard, it's not urgent but
> you're the guru with this stuff and I'm hesitant to change it
> without a better opinion ... Simple band aid for the meanwhile
> could be leave it as is but just disable counting if
> record/replay is in use.

When we unwind, we know how many insns remain in the tb.
With icount, we adjust cpu->neg.icount_decr.u16.low.

My suggestion is to change restore_state_to_opc to pass in either the raw insns_left, or 
the inverse: tb->icount - insns_left.

That'll be a trivial mechanical change for the signature of the hook, first.


r~