On Fri, Oct 30, 2026 at 2:56 PM Richard Henderson
<[email protected]> wrote:
> + if (tcg_ctx->gen_tb->cflags & CF_NO_GOTO_JC) {
> + tcg_gen_lookup_and_goto_ptr();
> + } else {
> + plugin_gen_disable_mem_helpers();
> + gen_jc_probe(pc, cs_base, flags);
> + }
This drops the CF_NO_GOTO_PTR check for migrated targets. Both the hit
path and the slow path in gen_jc_probe() end in goto_ptr, and
tcg_update_cflags() sets CF_NO_GOTO_PTR for gdb single-step without
CF_NO_GOTO_JC. cpu_exec_step_atomic() sets both, so only single-step is
exposed.
The result is that stepping over a branch doesn't stop if the
destination is already in the jump cache. With qemu-alpha -g, on a
200-iteration bsr/ret loop, one 's' packet on the bne ran the remaining
199 iterations. Patch 8 steps it correctly.
Honoring both flags fixes it, and the step trace then matches patch 8:
if (tcg_ctx->gen_tb->cflags & (CF_NO_GOTO_JC | CF_NO_GOTO_PTR)) {
Second problem: a probe hit skips the log_cpu_exec() call in
helper_lookup_tb_ptr(), so -d exec and -d cpu lose every inline
dispatch. Same loop, counting Trace lines:
-d exec 205 at patch 8, 6 at patch 14
-d exec,nochain 602 at patch 8, 5 at patch 14
The second one hurts most, since nochain exists so that those logs show
every TB. Taking the logging case off the inline path restores both
counts:
if (unlikely(!QTAILQ_EMPTY(&cpu->breakpoints) ||
qemu_loglevel_mask(CPU_LOG_TB_CPU | CPU_LOG_EXEC))) {
cflags |= CF_NO_GOTO_JC;
}
The monitor path already calls tcg_update_all_cflags() after changing
the log mask, so it stays correct. The comment there only mentions
CPU_LOG_TB_NOCHAIN and would want updating.
The commit message needs a pass too, and most of this is mine:
- "What is left is one_insn_per_tb and -d nochain; see below." There's
nothing below. one_insn_per_tb is fine as it is, and the logging case
is the part that needed covering.
- "the flags, cflags and cs_base ... are constants at translation
time" is no longer true of cs_base with goto_jc3.
- The last paragraph calls a backend opcode "a separate series", but
it's patches 10-13.
