[PATCH 18/23] plugins: add an API to read registers

[Alex Bennée]

We can only request a list of registers once the vCPU has been
initialised so the user needs to use either call the get function on
vCPU initialisation or during the translation phase.

We don't expose the reg number to the plugin instead hiding it behind
an opaque handle. This allows for a bit of future proofing should the
internals need to be changed while also being hashed against the
CPUClass so we can handle different register sets per-vCPU in
hetrogenous situations.

Having an internal state within the plugins also allows us to expand
the interface in future (for example providing callbacks on register
change if the translator can track changes).

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1706
Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20240103173349.398526-39-alex.bennee@linaro.org>
Based-on: <20231025093128.33116-18-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

---
v4
  - the get/read_registers functions are now implicitly for current
  vCPU only to accidental cpu != current_cpu uses.
---
 include/qemu/qemu-plugin.h   |  48 +++++++++++++++-
 plugins/api.c                | 107 +++++++++++++++++++++++++++++++++++
 plugins/qemu-plugins.symbols |   2 +
 3 files changed, 155 insertions(+), 2 deletions(-)

diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
index 93981f8f89f..3b6b18058d2 100644
--- a/include/qemu/qemu-plugin.h
+++ b/include/qemu/qemu-plugin.h
@@ -11,6 +11,7 @@
 #ifndef QEMU_QEMU_PLUGIN_H
 #define QEMU_QEMU_PLUGIN_H
 
+#include <glib.h>
 #include <inttypes.h>
 #include <stdbool.h>
 #include <stddef.h>
@@ -229,8 +230,8 @@ struct qemu_plugin_insn;
  * @QEMU_PLUGIN_CB_R_REGS: callback reads the CPU's regs
  * @QEMU_PLUGIN_CB_RW_REGS: callback reads and writes the CPU's regs
  *
- * Note: currently unused, plugins cannot read or change system
- * register state.
+ * Note: currently QEMU_PLUGIN_CB_RW_REGS is unused, plugins cannot change
+ * system register state.
  */
 enum qemu_plugin_cb_flags {
     QEMU_PLUGIN_CB_NO_REGS,
@@ -707,4 +708,47 @@ uint64_t qemu_plugin_end_code(void);
 QEMU_PLUGIN_API
 uint64_t qemu_plugin_entry_code(void);
 
+/** struct qemu_plugin_register - Opaque handle for register access */
+struct qemu_plugin_register;
+
+/**
+ * typedef qemu_plugin_reg_descriptor - register descriptions
+ *
+ * @handle: opaque handle for retrieving value with qemu_plugin_read_register
+ * @name: register name
+ * @feature: optional feature descriptor, can be NULL
+ */
+typedef struct {
+    struct qemu_plugin_register *handle;
+    const char *name;
+    const char *feature;
+} qemu_plugin_reg_descriptor;
+
+/**
+ * qemu_plugin_get_registers() - return register list for current vCPU
+ *
+ * Returns a GArray of qemu_plugin_reg_descriptor or NULL. Caller
+ * frees the array (but not the const strings).
+ *
+ * Should be used from a qemu_plugin_register_vcpu_init_cb() callback
+ * after the vCPU is initialised, i.e. in the vCPU context.
+ */
+GArray *qemu_plugin_get_registers(void);
+
+/**
+ * qemu_plugin_read_register() - read register for current vCPU
+ *
+ * @handle: a @qemu_plugin_reg_handle handle
+ * @buf: A GByteArray for the data owned by the plugin
+ *
+ * This function is only available in a context that register read access is
+ * explicitly requested via the QEMU_PLUGIN_CB_R_REGS flag.
+ *
+ * Returns the size of the read register. The content of @buf is in target byte
+ * order. On failure returns -1
+ */
+int qemu_plugin_read_register(struct qemu_plugin_register *handle,
+                              GByteArray *buf);
+
+
 #endif /* QEMU_QEMU_PLUGIN_H */
diff --git a/plugins/api.c b/plugins/api.c
index 54df72c1c00..483f04e85e4 100644
--- a/plugins/api.c
+++ b/plugins/api.c
@@ -8,6 +8,7 @@
  *
  *  qemu_plugin_tb
  *  qemu_plugin_insn
+ *  qemu_plugin_register
  *
  * Which can then be passed back into the API to do additional things.
  * As such all the public functions in here are exported in
@@ -35,10 +36,12 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/main-loop.h"
 #include "qemu/plugin.h"
 #include "qemu/log.h"
 #include "tcg/tcg.h"
 #include "exec/exec-all.h"
+#include "exec/gdbstub.h"
 #include "exec/ram_addr.h"
 #include "disas/disas.h"
 #include "plugin.h"
@@ -410,3 +413,107 @@ uint64_t qemu_plugin_entry_code(void)
 #endif
     return entry;
 }
+
+/*
+ * Register handles
+ *
+ * The plugin infrastructure keeps hold of these internal data
+ * structures which are presented to plugins as opaque handles. They
+ * are global to the system and therefor additions to the hash table
+ * must be protected by the @reg_handle_lock.
+ *
+ * In order to future proof for up-coming heterogeneous work we want
+ * different entries for each CPU type while sharing them in the
+ * common case of multiple cores of the same type.
+ */
+
+static QemuMutex reg_handle_lock;
+
+struct qemu_plugin_register {
+    const char *name;
+    int gdb_reg_num;
+};
+
+static GHashTable *reg_handles; /* hash table of PluginReg */
+
+/* Generate a stable key - would xxhash be overkill? */
+static gpointer cpu_plus_reg_to_key(CPUState *cs, int gdb_regnum)
+{
+    uintptr_t key = (uintptr_t) cs->cc;
+    key ^= gdb_regnum;
+    return GUINT_TO_POINTER(key);
+}
+
+/*
+ * Create register handles.
+ *
+ * We need to create a handle for each register so the plugin
+ * infrastructure can call gdbstub to read a register. We also
+ * construct a result array with those handles and some ancillary data
+ * the plugin might find useful.
+ */
+
+static GArray *create_register_handles(CPUState *cs, GArray *gdbstub_regs)
+{
+    GArray *find_data = g_array_new(true, true,
+                                    sizeof(qemu_plugin_reg_descriptor));
+
+    WITH_QEMU_LOCK_GUARD(&reg_handle_lock) {
+
+        if (!reg_handles) {
+            reg_handles = g_hash_table_new(g_direct_hash, g_direct_equal);
+        }
+
+        for (int i = 0; i < gdbstub_regs->len; i++) {
+            GDBRegDesc *grd = &g_array_index(gdbstub_regs, GDBRegDesc, i);
+            gpointer key = cpu_plus_reg_to_key(cs, grd->gdb_reg);
+            struct qemu_plugin_register *val = g_hash_table_lookup(reg_handles,
+                                                                   key);
+
+            /* skip "un-named" regs */
+            if (!grd->name) {
+                continue;
+            }
+
+            /* Doesn't exist, create one */
+            if (!val) {
+                val = g_new0(struct qemu_plugin_register, 1);
+                val->gdb_reg_num = grd->gdb_reg;
+                val->name = g_intern_string(grd->name);
+
+                g_hash_table_insert(reg_handles, key, val);
+            }
+
+            /* Create a record for the plugin */
+            qemu_plugin_reg_descriptor desc = {
+                .handle = val,
+                .name = val->name,
+                .feature = g_intern_string(grd->feature_name)
+            };
+            g_array_append_val(find_data, desc);
+        }
+    }
+
+    return find_data;
+}
+
+GArray *qemu_plugin_get_registers(void)
+{
+    g_assert(current_cpu);
+
+    g_autoptr(GArray) regs = gdb_get_register_list(current_cpu);
+    return regs->len ? create_register_handles(current_cpu, regs) : NULL;
+}
+
+int qemu_plugin_read_register(struct qemu_plugin_register *reg, GByteArray 
*buf)
+{
+    g_assert(current_cpu);
+
+    return gdb_read_register(current_cpu, buf, reg->gdb_reg_num);
+}
+
+static void __attribute__((__constructor__)) qemu_api_init(void)
+{
+    qemu_mutex_init(&reg_handle_lock);
+
+}
diff --git a/plugins/qemu-plugins.symbols b/plugins/qemu-plugins.symbols
index adb67608598..27fe97239be 100644
--- a/plugins/qemu-plugins.symbols
+++ b/plugins/qemu-plugins.symbols
@@ -3,6 +3,7 @@
   qemu_plugin_end_code;
   qemu_plugin_entry_code;
   qemu_plugin_get_hwaddr;
+  qemu_plugin_get_registers;
   qemu_plugin_hwaddr_device_name;
   qemu_plugin_hwaddr_is_io;
   qemu_plugin_hwaddr_phys_addr;
@@ -19,6 +20,7 @@
   qemu_plugin_num_vcpus;
   qemu_plugin_outs;
   qemu_plugin_path_to_binary;
+  qemu_plugin_read_register;
   qemu_plugin_register_atexit_cb;
   qemu_plugin_register_flush_cb;
   qemu_plugin_register_vcpu_exit_cb;
-- 
2.39.2