arm: Clarify the logic of set_pc()

Until now, the set_pc logic was unclear, which raised questions about
whether it should be used directly, applying a value to PC or adding
additional checks, for example, set the Thumb bit in Arm cpu. Let's set
the set_pc logic for “Configure the PC, as was done in the ELF file”
and implement synchronize_with_tb hook for preserving PC to cpu_tb_exec.

Signed-off-by: Julia Suvorova <jusual@mail.ru>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190129121817.7109-1-jusual@mail.ru
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 4c2feb9c17..1d6099e5d4 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -103,9 +103,21 @@ struct TranslationBlock;
  * @get_arch_id: Callback for getting architecture-dependent CPU ID.
  * @get_paging_enabled: Callback for inquiring whether paging is enabled.
  * @get_memory_mapping: Callback for obtaining the memory mappings.
- * @set_pc: Callback for setting the Program Counter register.
+ * @set_pc: Callback for setting the Program Counter register. This
+ *       should have the semantics used by the target architecture when
+ *       setting the PC from a source such as an ELF file entry point;
+ *       for example on Arm it will also set the Thumb mode bit based
+ *       on the least significant bit of the new PC value.
+ *       If the target behaviour here is anything other than "set
+ *       the PC register to the value passed in" then the target must
+ *       also implement the synchronize_from_tb hook.
  * @synchronize_from_tb: Callback for synchronizing state from a TCG
- * #TranslationBlock.
+ *       #TranslationBlock. This is called when we abandon execution
+ *       of a TB before starting it, and must set all parts of the CPU
+ *       state which the previous TB in the chain may not have updated.
+ *       This always includes at least the program counter; some targets
+ *       will need to do more. If this hook is not implemented then the
+ *       default is to call @set_pc(tb->pc).
  * @handle_mmu_fault: Callback for handling an MMU fault.
  * @get_phys_page_debug: Callback for obtaining a physical address.
  * @get_phys_page_attrs_debug: Callback for obtaining a physical address and the