Re: [Qemu-devel] [PATCH v6 00/25] tcg cross-tb optimizations

[Emilio G. Cota]

On Tue, May 02, 2017 at 20:36:52 -0700, Richard Henderson wrote:
> On 05/02/2017 12:22 PM, Richard Henderson wrote:
> >Changes since v5:
> ...
> >   * Alpha frontend patch rewritten; the former patch appears to
> >     drop clock interrupts, not exiting the kernel's idle loop.
> >     I never *really* figured out why, since both patches seem
> >     to annotate the same TBs in the same way.
> 
> There's definitely something odd going on.
> 
> With a rebuild from scratch, the same symptoms have re-appeared for Alpha.
> So it really had nothing to do with the original patch.  I'm at a bit of a
> loss...

I can reliably reproduce a freeze upon booting.

Not sure this can help much (this is the first time I run an Alpha
guest), but here are some findings.

In my testing, if I disable the lookup for JMP/JSR/ret, I can boot OK.
This works:

+++ b/target/alpha/translate.c
@@ -2435,12 +2435,16 @@ static ExitStatus translate_one(DisasContext *ctx, 
uint32_t insn)
         if (ra != 31) {
             tcg_gen_movi_i64(ctx->ir[ra], ctx->pc);
         }
+#if 0
         if (use_exit_tb(ctx)) {
             ret = EXIT_PC_UPDATED;
         } else {
             tcg_gen_lookup_and_goto_ptr(cpu_pc);
             ret = EXIT_GOTO_TB;
         }
+#else
+        ret = EXIT_PC_UPDATED;
+#endif
         break;

However, this doesn't tell us much, since these jumps are pretty common.

Interestingly, if I leave the lookup_and_goto_ptr above (s/#if 0/#if 1/), but
change the lookup_ptr helper to bypass tb_jmp_cache and directly check the
htable, it boots OK.

Could it be that we're forgetting to clear (or set) tb_jmp_cache somewhere?

                Emilio