plugins: fix optimization in plugin_gen_disable_mem_helpers

We were mistakenly checking tcg_ctx->plugin_insn as a canary to know
whether the TB had emitted helpers that might have accessed memory.

The problem is that tcg_ctx->plugin_insn gets updated on every
instruction in the TB, which results in us wrongly performing the
optimization (i.e. not clearing cpu->plugin_mem_cbs) way too often,
since it's not rare that the last instruction in the TB doesn't
use helpers.

Fix it by tracking a per-TB canary.

While at it, expand documentation.

Related: #1381

Signed-off-by: Emilio Cota <cota@braap.org>
Message-Id: <20230108164731.61469-2-cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230124180127.1881110-32-alex.bennee@linaro.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
index a772e14193..e0ebedef84 100644
--- a/include/qemu/plugin.h
+++ b/include/qemu/plugin.h
@@ -118,7 +118,10 @@ struct qemu_plugin_insn {
     void *haddr;
     GArray *cbs[PLUGIN_N_CB_TYPES][PLUGIN_N_CB_SUBTYPES];
     bool calls_helpers;
+
+    /* if set, the instruction calls helpers that might access guest memory */
     bool mem_helper;
+
     bool mem_only;
 };
 
@@ -158,6 +161,10 @@ struct qemu_plugin_tb {
     void *haddr1;
     void *haddr2;
     bool mem_only;
+
+    /* if set, the TB calls helpers that might access guest memory */
+    bool mem_helper;
+
     GArray *cbs[PLUGIN_N_CB_SUBTYPES];
 };