[RFC][PATCH] accel/tcg: Use lookup_and_goto_ptr() for linux-user in translator_use_goto_tb()

[Helge Deller <deller@gmx.de>]

I'm quite unclear about translator_use_goto_tb() for qemu-user
emulation....(and in general).

Based on the function name, the function translator_use_goto_tb() shall
help to decide if a program should use goto_tb() and exit_tb() to jump
to the next instruction.

Currently, if the destination is on the same page, it returns true.
I wonder, if it shouldn't return false in this case instead, because
arches have code like this: (taken from target/hppa/translate.c):
    if (... && translator_use_goto_tb(ctx, f)) {
        tcg_gen_goto_tb(which);
        tcg_gen_movi_reg(cpu_iaoq_f, f);
        tcg_gen_movi_reg(cpu_iaoq_b, b);
        tcg_gen_exit_tb(ctx->base.tb, which);
    } else {
        copy_iaoq_entry(cpu_iaoq_f, f, cpu_iaoq_b);
        copy_iaoq_entry(cpu_iaoq_b, b, ctx->iaoq_n_var);
        tcg_gen_lookup_and_goto_ptr();
    }

Shouldn't, if the destination is on the same page, the (faster?)
path with tcg_gen_lookup_and_goto_ptr() be taken instead?

Now, for user-mode emulation page faults can't happen at all.
Shouldn't in this case the tcg_gen_lookup_and_goto_ptr() path been taken
unconditionally, as shown in the patch below?

Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
index 1a6a5448c8..07224a7f83 100644
--- a/accel/tcg/translator.c
+++ b/accel/tcg/translator.c
@@ -124,8 +124,13 @@ bool translator_use_goto_tb(DisasContextBase *db, vaddr dest)
         return false;
     }

+#ifndef CONFIG_USER_ONLY
     /* Check for the dest on the same page as the start of the TB.  */
     return ((db->pc_first ^ dest) & TARGET_PAGE_MASK) == 0;
+#else
+    /* linux-user doesn't need to fear pagefaults for exec swap-in */
+    return false;
+#endif
 }

 void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,