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

["Nicholas Piggin" <npiggin@gmail.com>]

On Thu May 23, 2024 at 8:46 AM AEST, Richard Henderson wrote:
> 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.

That gives me a better place to start looking.

Thanks,
Nick