TCG plugin to keep track what byte is load/store into memory

Hi I want to write a TCG plugin to keep track what byte is load/store into memory. But the vcpu_mem stub seems only has the address, but not the value I store/load. How can I do that?

static void vcpu_mem(unsigned int cpu_index, qemu_plugin_meminfo_t meminfo, uint64_t vaddr, void *udata)

thanks Peter