On 8/26/26 22:02, Matt Turner wrote:
curr_cflags() is called once per TB dispatch, from helper_lookup_tb_ptr() and from the cpu_exec() loop. It recomputes the same value every time:uint32_t cflags = cpu->tcg_cflags; if (unlikely(cpu_single_stepping(cpu))) { ... } else if (qatomic_read(&one_insn_per_tb)) { ... } else if (qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) { ... } That is three loads and three branches on the hottest path in the interpreter, for state that changes only when gdb enables single-step, when one-insn-per-tb is toggled, or when the log mask changes. None of the three has to be sampled at dispatch time. Fold each into CPUState::tcg_cflags where it changes and curr_cflags() becomes a single load of a field that TB lookup has to read anyway. The derived bits -- CF_COUNT_MASK, CF_NO_GOTO_TB, CF_NO_GOTO_PTR and CF_SINGLE_STEP -- are never set by tcg_cflags_set(), so tcg_update_cflags() can recompute them in place without disturbing the rest, and conversely tcg_cflags_set() ORs in its bits without disturbing them. There are three places to call it: - tcg_exec_realizefn(), so that a CPU created after the command line has been parsed starts out with the right value. This covers user-only, where tcg_cpu_init_cflags() is not reached. linux-user's cpu_copy() copies tcg_cflags wholesale, so a cloned thread inherits it. - cpu_single_step(), which changes one CPU. gdb is the only caller that matters; in system mode it runs with the vCPUs stopped, and in user mode gdb_continue_partial() can reach a thread that is still running, because gdb_handlesig() stops only the thread that trapped. That is exactly the plain cross-thread store to another CPU's CPUState that cpu->singlestep_flags already was, read back by that CPU through cpu_single_stepping() in curr_cflags(). This patch changes which field carries it, not who writes it or how. - hmp_one_insn_per_tb() and hmp_log(), which change every CPU while the vCPUs are running, so the update is queued with async_run_on_cpu() and each CPU writes its own cflags from its own thread. The command line spellings of those two settings need nothing: they are parsed before any CPU is realized, so tcg_exec_realizefn() picks them up. Measured with qemu-alpha running an emulated alpha gcc 16.2.0 compiling the SQLite 3.45.1 amalgamation (255k lines, -O2) on an x86-64 host, in a build configured with --enable-lto: before: 1,646,994,254,249 instructions after: 1,562,204,796,597 instructions -5.15% That workload issues 8.4 billion dispatches, so the per-call saving is small but the aggregate is not. The emulated compiler produces byte-identical output before and after. Wall clock does not move: 133.19s to 132.58s, a 0.46% difference against a run-to-run spread larger than that. The removed work is a few predictable loads and branches that the host executes largely in parallel with the surrounding dispatch, so this patch is worth taking for the instruction count and for what it enables, not for a time saving that can be measured on its own. v4: Update the cflags from the HMP handlers for 'log' and 'one-insn-per-tb' rather than from qemu_set_log_internal() and the accelerator property setter. Those are the paths that reach a running vCPU, and the monitor is the only thing that does. Suggested by Richard Henderson. v4: Queue the per-CPU update with async_run_on_cpu() rather than async_safe_run_on_cpu(). Halting the other vCPUs buys nothing: the queued work already runs on the owning CPU's own thread. Suggested by Alex Bennee, who also asked whether there are cross-vCPU updates of tcg_cflags at all. With this change the monitor path has none: the only remaining writer from another thread is cpu_single_step(), above, which is neither new nor made worse here. v4: Move the stub to accel/stubs/, which is where the other accelerator stubs live. Signed-off-by: Matt Turner<[email protected]> --- accel/stubs/meson.build | 1 + accel/stubs/tcg-stub.c | 16 ++++++++++++++++ accel/tcg/cpu-exec-common.c | 33 ++++++++++++++++++++++++++++++--- accel/tcg/cpu-exec.c | 3 +++ accel/tcg/internal-common.h | 11 +++++++++-- cpu-target.c | 3 +++ include/system/tcg.h | 12 ++++++++++++ monitor/hmp-cmds.c | 5 +++++ system/runstate-hmp-cmds.c | 4 ++++ 9 files changed, 83 insertions(+), 5 deletions(-) create mode 100644 accel/stubs/tcg-stub.c
Reviewed-by: Richard Henderson <[email protected]> r~
