[PULL 26/28] plugins: move reset of plugin data to tb_start

["Alex Bennée" <alex.bennee@linaro.org>]

We can't always guarantee we get to the end of a translator loop.
Although this can happen for a variety of reasons it does happen more
often on x86 system emulation when an instruction spans across to an
un-faulted page. This caused confusion of the instruction tracking
data resulting in apparent reverse execution (at least from the
plugins point of view).

Fix this by moving the reset code to plugin_gen_tb_start so we always
start with a clean slate.

We unconditionally reset tcg_ctx->plugin_insn as the
plugin_insn_append code uses this as a proxy for knowing if plugins
are enabled for the current instruction. Otherwise we can hit a race
where a previously instrumented thread leaves a stale value after the
main thread exits and disables instrumentation.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/824
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220204204335.1689602-27-alex.bennee@linaro.org>

diff --git a/accel/tcg/plugin-gen.c b/accel/tcg/plugin-gen.c
index 22d95fe1c3..3d0b101e34 100644
--- a/accel/tcg/plugin-gen.c
+++ b/accel/tcg/plugin-gen.c
@@ -854,10 +854,20 @@ static void plugin_gen_inject(const struct qemu_plugin_tb *plugin_tb)
 
 bool plugin_gen_tb_start(CPUState *cpu, const TranslationBlock *tb, bool mem_only)
 {
-    struct qemu_plugin_tb *ptb = tcg_ctx->plugin_tb;
     bool ret = false;
 
     if (test_bit(QEMU_PLUGIN_EV_VCPU_TB_TRANS, cpu->plugin_mask)) {
+        struct qemu_plugin_tb *ptb = tcg_ctx->plugin_tb;
+        int i;
+
+        /* reset callbacks */
+        for (i = 0; i < PLUGIN_N_CB_SUBTYPES; i++) {
+            if (ptb->cbs[i]) {
+                g_array_set_size(ptb->cbs[i], 0);
+            }
+        }
+        ptb->n = 0;
+
         ret = true;
 
         ptb->vaddr = tb->pc;
@@ -868,6 +878,9 @@ bool plugin_gen_tb_start(CPUState *cpu, const TranslationBlock *tb, bool mem_onl
 
         plugin_gen_empty_callback(PLUGIN_GEN_FROM_TB);
     }
+
+    tcg_ctx->plugin_insn = NULL;
+
     return ret;
 }
 
@@ -904,23 +917,19 @@ void plugin_gen_insn_end(void)
     plugin_gen_empty_callback(PLUGIN_GEN_AFTER_INSN);
 }
 
+/*
+ * There are cases where we never get to finalise a translation - for
+ * example a page fault during translation. As a result we shouldn't
+ * do any clean-up here and make sure things are reset in
+ * plugin_gen_tb_start.
+ */
 void plugin_gen_tb_end(CPUState *cpu)
 {
     struct qemu_plugin_tb *ptb = tcg_ctx->plugin_tb;
-    int i;
 
     /* collect instrumentation requests */
     qemu_plugin_tb_trans_cb(cpu, ptb);
 
     /* inject the instrumentation at the appropriate places */
     plugin_gen_inject(ptb);
-
-    /* clean up */
-    for (i = 0; i < PLUGIN_N_CB_SUBTYPES; i++) {
-        if (ptb->cbs[i]) {
-            g_array_set_size(ptb->cbs[i], 0);
-        }
-    }
-    ptb->n = 0;
-    tcg_ctx->plugin_insn = NULL;
 }
-- 
2.30.2