Re: [PATCH 12/19] target/ppc/pmu_book3s_helper.c: enable PMC1 counter negative EBB

[Richard Henderson]

On 8/11/21 6:45 PM, Richard Henderson wrote:
> On 8/11/21 5:39 PM, David Gibson wrote:
> > > I mean, nothing is stopping us from calculating cycles using time, but in the
> > > end we would do the same thing we're already doing today.
> >
> > Oh.. ok.  I had assumed that icount worked by instrumenting the
> > generate TCG code to actually count instructions, rather than working
> > off the time.
>
> David, you're right, icount instruments the generated tcg code.
> You also have to add -icount to the command-line.

Oh, and btw, icount disables multi-threaded tcg, so you're going to be running that guest 
in round-robin mode.

Icount affects so many aspects of qemu that I really do not think it is the best option 
for a PMU.

If you want to count instructions retired, then just do that.  Stuff values into 
tcg_gen_insn_start so they're available for exception unwind, and otherwise decrement the 
counter at the end of a TB.

If you really must interrupt exactly at 0 (and not simply at some point past underflow), 
then we can adjust the tb lookup logic to let you reduce tb->cflags.CF_COUNT_MASK in 
cpu_get_tb_cpu_state.


r~