[PULL 20/22] contrib/plugins: optimise the register value tracking

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

This adds an additional flag which attempts to optimise the register
tracking by only instrumenting instructions which are likely to change
its value. This relies on the disassembler showing up the register
names in disassembly so is only enabled when asked for.

Message-Id: <20240103173349.398526-42-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

diff --git a/docs/devel/tcg-plugins.rst b/docs/devel/tcg-plugins.rst
index 3a0962723d7..fa7421279f5 100644
--- a/docs/devel/tcg-plugins.rst
+++ b/docs/devel/tcg-plugins.rst
@@ -503,7 +503,15 @@ registers with multiple ``reg`` options. You can also use glob style matching if
   $ qemu-system-arm $(QEMU_ARGS) \
     -plugin ./contrib/plugins/libexeclog.so,reg=\*_el2,reg=sp -d plugin
 
-Be aware that each additional register to check will slow down execution quite considerably.
+Be aware that each additional register to check will slow down
+execution quite considerably. You can optimise the number of register
+checks done by using the rdisas option. This will only instrument
+instructions that mention the registers in question in disassembly.
+This is not foolproof as some instructions implicitly change
+instructions. You can use the ifilter to catch these cases:
+
+  $ qemu-system-arm $(QEMU_ARGS) \
+    -plugin ./contrib/plugins/libexeclog.so,ifilter=msr,ifilter=blr,reg=x30,reg=\*_el1,rdisas=on
 
 - contrib/plugins/cache.c
 
diff --git a/contrib/plugins/execlog.c b/contrib/plugins/execlog.c
index c20e88a6941..5a4de1c93be 100644
--- a/contrib/plugins/execlog.c
+++ b/contrib/plugins/execlog.c
@@ -27,6 +27,7 @@ typedef struct CPU {
     GString *last_exec;
     /* Ptr array of Register */
     GPtrArray *registers;
+    int index;
 } CPU;
 
 QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
@@ -38,6 +39,9 @@ static GRWLock expand_array_lock;
 static GPtrArray *imatches;
 static GArray *amatches;
 static GPtrArray *rmatches;
+static bool disas_assist;
+static GMutex add_reg_name_lock;
+static GPtrArray *all_reg_names;
 
 /**
  * Add memory read or write information to current instruction log
@@ -72,9 +76,14 @@ static void vcpu_mem(unsigned int cpu_index, qemu_plugin_meminfo_t info,
 }
 
 /**
- * Log instruction execution
+ * Log instruction execution, outputting the last one.
+ *
+ * vcpu_insn_exec() is a copy and paste of vcpu_insn_exec_with_regs()
+ * without the checking of register values when we've attempted to
+ * optimise with disas_assist.
  */
-static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
+
+static CPU *get_cpu(int cpu_index)
 {
     CPU *cpu;
 
@@ -83,29 +92,42 @@ static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
     cpu = &cpus[cpu_index];
     g_rw_lock_reader_unlock(&expand_array_lock);
 
+    return cpu;
+}
+
+static void insn_check_regs(CPU *cpu)
+{
+    for (int n = 0; n < cpu->registers->len; n++) {
+        Register *reg = cpu->registers->pdata[n];
+        int sz;
+
+        g_byte_array_set_size(reg->new, 0);
+        sz = qemu_plugin_read_register(cpu->index, reg->handle, reg->new);
+        g_assert(sz == reg->last->len);
+
+        if (memcmp(reg->last->data, reg->new->data, sz)) {
+            GByteArray *temp = reg->last;
+            g_string_append_printf(cpu->last_exec, ", %s -> 0x", reg->name);
+            /* TODO: handle BE properly */
+            for (int i = sz; i >= 0; i--) {
+                g_string_append_printf(cpu->last_exec, "%02x",
+                                       reg->new->data[i]);
+            }
+            reg->last = reg->new;
+            reg->new = temp;
+        }
+    }
+}
+
+/* Log last instruction while checking registers */
+static void vcpu_insn_exec_with_regs(unsigned int cpu_index, void *udata)
+{
+    CPU *cpu = get_cpu(cpu_index);
+
     /* Print previous instruction in cache */
     if (cpu->last_exec->len) {
         if (cpu->registers) {
-            for (int n = 0; n < cpu->registers->len; n++) {
-                Register *reg = cpu->registers->pdata[n];
-                int sz;
-
-                g_byte_array_set_size(reg->new, 0);
-                sz = qemu_plugin_read_register(cpu_index, reg->handle, reg->new);
-                g_assert(sz == reg->last->len);
-
-                if (memcmp(reg->last->data, reg->new->data, sz)) {
-                    GByteArray *temp = reg->last;
-                    g_string_append_printf(cpu->last_exec, ", %s -> 0x", reg->name);
-                    /* TODO: handle BE properly */
-                    for (int i = sz; i >= 0; i--) {
-                        g_string_append_printf(cpu->last_exec, "%02x",
-                                               reg->new->data[i]);
-                    }
-                    reg->last = reg->new;
-                    reg->new = temp;
-                }
-            }
+            insn_check_regs(cpu);
         }
 
         qemu_plugin_outs(cpu->last_exec->str);
@@ -114,8 +136,44 @@ static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
 
     /* Store new instruction in cache */
     /* vcpu_mem will add memory access information to last_exec */
-    g_string_printf(cpus[cpu_index].last_exec, "%u, ", cpu_index);
-    g_string_append(cpus[cpu_index].last_exec, (char *)udata);
+    g_string_printf(cpu->last_exec, "%u, ", cpu_index);
+    g_string_append(cpu->last_exec, (char *)udata);
+}
+
+/* Log last instruction while checking registers, ignore next */
+static void vcpu_insn_exec_only_regs(unsigned int cpu_index, void *udata)
+{
+    CPU *cpu = get_cpu(cpu_index);
+
+    /* Print previous instruction in cache */
+    if (cpu->last_exec->len) {
+        if (cpu->registers) {
+            insn_check_regs(cpu);
+        }
+
+        qemu_plugin_outs(cpu->last_exec->str);
+        qemu_plugin_outs("\n");
+    }
+
+    /* reset */
+    cpu->last_exec->len = 0;
+}
+
+/* Log last instruction without checking regs, setup next */
+static void vcpu_insn_exec(unsigned int cpu_index, void *udata)
+{
+    CPU *cpu = get_cpu(cpu_index);
+
+    /* Print previous instruction in cache */
+    if (cpu->last_exec->len) {
+        qemu_plugin_outs(cpu->last_exec->str);
+        qemu_plugin_outs("\n");
+    }
+
+    /* Store new instruction in cache */
+    /* vcpu_mem will add memory access information to last_exec */
+    g_string_printf(cpu->last_exec, "%u, ", cpu_index);
+    g_string_append(cpu->last_exec, (char *)udata);
 }
 
 /**
@@ -128,6 +186,8 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
 {
     struct qemu_plugin_insn *insn;
     bool skip = (imatches || amatches);
+    bool check_regs_this = rmatches;
+    bool check_regs_next = false;
 
     size_t n = qemu_plugin_tb_n_insns(tb);
     for (size_t i = 0; i < n; i++) {
@@ -148,7 +208,8 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
         /*
          * If we are filtering we better check out if we have any
          * hits. The skip "latches" so we can track memory accesses
-         * after the instruction we care about.
+         * after the instruction we care about. Also enable register
+         * checking on the next instruction.
          */
         if (skip && imatches) {
             int j;
@@ -156,6 +217,7 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
                 char *m = g_ptr_array_index(imatches, j);
                 if (g_str_has_prefix(insn_disas, m)) {
                     skip = false;
+                    check_regs_next = rmatches;
                 }
             }
         }
@@ -170,8 +232,39 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
             }
         }
 
+        /*
+         * Check the disassembly to see if a register we care about
+         * will be affected by this instruction. This relies on the
+         * dissembler doing something sensible for the registers we
+         * care about.
+         */
+        if (disas_assist && rmatches) {
+            check_regs_next = false;
+            gchar *args = g_strstr_len(insn_disas, -1, " ");
+            for (int n = 0; n < all_reg_names->len; n++) {
+                gchar *reg = g_ptr_array_index(all_reg_names, n);
+                if (g_strrstr(args, reg)) {
+                    check_regs_next = true;
+                    skip = false;
+                }
+            }
+        }
+
+        /*
+         * We now have 3 choices:
+         *
+         * - Log insn
+         * - Log insn while checking registers
+         * - Don't log this insn but check if last insn changed registers
+         */
+
         if (skip) {
-            g_free(insn_disas);
+            if (check_regs_this) {
+                qemu_plugin_register_vcpu_insn_exec_cb(insn,
+                                                       vcpu_insn_exec_only_regs,
+                                                       QEMU_PLUGIN_CB_R_REGS,
+                                                       NULL);
+            }
         } else {
             uint32_t insn_opcode;
             insn_opcode = *((uint32_t *)qemu_plugin_insn_data(insn));
@@ -184,15 +277,28 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
                                              QEMU_PLUGIN_MEM_RW, NULL);
 
             /* Register callback on instruction */
-            qemu_plugin_register_vcpu_insn_exec_cb(
-                insn, vcpu_insn_exec,
-                rmatches ? QEMU_PLUGIN_CB_R_REGS : QEMU_PLUGIN_CB_NO_REGS,
-                output);
+            if (check_regs_this) {
+                qemu_plugin_register_vcpu_insn_exec_cb(
+                    insn, vcpu_insn_exec_with_regs,
+                    QEMU_PLUGIN_CB_R_REGS,
+                    output);
+            } else {
+                qemu_plugin_register_vcpu_insn_exec_cb(
+                    insn, vcpu_insn_exec,
+                    QEMU_PLUGIN_CB_NO_REGS,
+                    output);
+            }
 
             /* reset skip */
             skip = (imatches || amatches);
         }
 
+        /* set regs for next */
+        if (disas_assist && rmatches) {
+            check_regs_this = check_regs_next;
+        }
+
+        g_free(insn_disas);
     }
 }
 
@@ -200,10 +306,11 @@ static Register *init_vcpu_register(int vcpu_index,
                                     qemu_plugin_reg_descriptor *desc)
 {
     Register *reg = g_new0(Register, 1);
+    g_autofree gchar *lower = g_utf8_strdown(desc->name, -1);
     int r;
 
     reg->handle = desc->handle;
-    reg->name = g_strdup(desc->name);
+    reg->name = g_intern_string(lower);
     reg->last = g_byte_array_new();
     reg->new = g_byte_array_new();
 
@@ -213,7 +320,7 @@ static Register *init_vcpu_register(int vcpu_index,
     return reg;
 }
 
-static registers_init(int vcpu_index)
+static void registers_init(int vcpu_index)
 {
     GPtrArray *registers = g_ptr_array_new();
     g_autoptr(GArray) reg_list = qemu_plugin_get_registers(vcpu_index);
@@ -228,9 +335,20 @@ static registers_init(int vcpu_index)
                 reg_list, qemu_plugin_reg_descriptor, r);
             for (int p = 0; p < rmatches->len; p++) {
                 g_autoptr(GPatternSpec) pat = g_pattern_spec_new(rmatches->pdata[p]);
-                if (g_pattern_match_string(pat, rd->name)) {
+                g_autofree gchar *rd_lower = g_utf8_strdown(rd->name, -1);
+                if (g_pattern_match_string(pat, rd->name) ||
+                    g_pattern_match_string(pat, rd_lower)) {
                     Register *reg = init_vcpu_register(vcpu_index, rd);
                     g_ptr_array_add(registers, reg);
+
+                    /* we need a list of regnames at TB translation time */
+                    if (disas_assist) {
+                        g_mutex_lock(&add_reg_name_lock);
+                        if (!g_ptr_array_find(all_reg_names, reg->name, NULL)) {
+                            g_ptr_array_add(all_reg_names, reg->name);
+                        }
+                        g_mutex_unlock(&add_reg_name_lock);
+                    }
                 }
             }
         }
@@ -254,6 +372,7 @@ static void vcpu_init(qemu_plugin_id_t id, unsigned int vcpu_index)
     if (vcpu_index >= num_cpus) {
         cpus = g_realloc_n(cpus, vcpu_index + 1, sizeof(*cpus));
         while (vcpu_index >= num_cpus) {
+            cpus[num_cpus].index = vcpu_index;
             cpus[num_cpus].last_exec = g_string_new(NULL);
 
             /* Any registers to track? */
@@ -336,6 +455,12 @@ QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
             parse_vaddr_match(tokens[1]);
         } else if (g_strcmp0(tokens[0], "reg") == 0) {
             add_regpat(tokens[1]);
+        } else if (g_strcmp0(tokens[0], "rdisas") == 0) {
+            if (!qemu_plugin_bool_parse(tokens[0], tokens[1], &disas_assist)) {
+                fprintf(stderr, "boolean argument parsing failed: %s\n", opt);
+                return -1;
+            }
+            all_reg_names = g_ptr_array_new();
         } else {
             fprintf(stderr, "option parsing failed: %s\n", opt);
             return -1;
-- 
2.39.2