plugins: save value during memory accesses

Different code paths handle memory accesses:
- tcg generated code
- load/store helpers
- atomic helpers

This value is saved in cpu->neg.plugin_mem_value_{high,low}. Values are
written only for accessed word size (upper bits are not set).

Atomic operations are doing read/write at the same time, so we generate
two memory callbacks instead of one, to allow plugins to access distinct
values.

For now, we can have access only up to 128 bits, thus split this in two
64 bits words. When QEMU will support wider operations, we'll be able to
reconsider this.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240724194708.1843704-2-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-5-alex.bennee@linaro.org>

include/hw/core/cpu.h

@@ -350,6 +350,8 @@ typedef union IcountDecr {
  *                         from CPUArchState, via small negative offsets.
  * @can_do_io: True if memory-mapped IO is allowed.
  * @plugin_mem_cbs: active plugin memory callbacks
+ * @plugin_mem_value_low: 64 lower bits of latest accessed mem value.
+ * @plugin_mem_value_high: 64 higher bits of latest accessed mem value.
  */
 typedef struct CPUNegativeOffsetState {
     CPUTLB tlb;
@@ -358,6 +360,8 @@ typedef struct CPUNegativeOffsetState {
      * The callback pointer are accessed via TCG (see gen_empty_mem_helper).
      */
     GArray *plugin_mem_cbs;
+    uint64_t plugin_mem_value_low;
+    uint64_t plugin_mem_value_high;
 #endif
     IcountDecr icount_decr;
     bool can_do_io;