On Thu, Jun 28, 2018 at 10:35:24PM +0200, Sebastian Bauer wrote: > When a branch instructions points to itself, only external events will > change the internally observable cpu state. This change will adjust the > behaviour of QEMU such that it sends the emulated cpu into a sleep state > if this case is detected. > > The effect for guests whose idle task contains this endless loop (e.g., > AmigaOS) is that they no longer hog the cpu of the host if they are idle. > > Signed-off-by: Sebastian Bauer <m...@sebastianbauer.info> > > --- > > This is a RFC because I'm not really familiar with the internals > of QEMU. There are probably other (better) ways to achive a similar > behaviour. For that reason I haven't tested the change thoroughly > yet, except that it produces the desired effect. > > The patch will change the behaviour wrt. the instruction count which > is no longer the same. But I don't think that this will matter. > > I will be grateful for any expressed concerns and suggestion of that > matter. > > Patch is based on ppc-for-3.0.
Doesn't look insane to me, though I'm really not a TCG expert.
I will note that its utility is probably pretty limited. I think most
OSes have a more complex idle loop than an unconditional branch to
self.
> ---
> target/ppc/helper.h | 2 ++
> target/ppc/translate.c | 19 +++++++++++++++++++
> 2 files changed, 21 insertions(+)
>
> diff --git a/target/ppc/helper.h b/target/ppc/helper.h
> index d751f0e219..5f9b157417 100644
> --- a/target/ppc/helper.h
> +++ b/target/ppc/helper.h
> @@ -799,3 +799,5 @@ DEF_HELPER_4(dscliq, void, env, fprp, fprp, i32)
>
> DEF_HELPER_1(tbegin, void, env)
> DEF_HELPER_FLAGS_1(fixup_thrm, TCG_CALL_NO_RWG, void, env)
> +
> +DEF_HELPER_1(sleep, void, env)
> diff --git a/target/ppc/translate.c b/target/ppc/translate.c
> index 3a215a1dc6..7db53003ea 100644
> --- a/target/ppc/translate.c
> +++ b/target/ppc/translate.c
> @@ -3469,6 +3469,18 @@ static inline void gen_setlr(DisasContext *ctx,
> target_ulong nip)
> tcg_gen_movi_tl(cpu_lr, nip);
> }
>
> +/* Helper for bringing the cpu to the sleep state so it will
> + * react only on "external" events */
> +void helper_sleep(CPUPPCState *env)
> +{
> + CPUState *cs = CPU(ppc_env_get_cpu(env));
> +
> + cs->exception_index = EXCP_HALTED;
> + cs->exit_request = 1;
> + cs->halted = 1;
> + cpu_loop_exit(cs);
> +}
> +
> /* b ba bl bla */
> static void gen_b(DisasContext *ctx)
> {
> @@ -3483,6 +3495,13 @@ static void gen_b(DisasContext *ctx)
> } else {
> target = li;
> }
> +
> + if (target == ctx->base.pc_next - 4) {
> + /* Endless-loop, CPU can now react on "external" events only, so we
> + * simply go to sleep in order to save host processing resources */
> + gen_helper_sleep(cpu_env);
> + }
> +
> if (LK(ctx->opcode)) {
> gen_setlr(ctx, ctx->base.pc_next);
> }
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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