target/arm: Implement BXNS, and banked stack pointers

Implement the BXNS v8M instruction, which is like BX but will do a
jump-and-switch-to-NonSecure if the branch target address has bit 0
clear.

This is the first piece of code which implements "switch to the
other security state", so the commit also includes the code to
switch the stack pointers around, which is the only complicated
part of switching security state.

BLXNS is more complicated than just "BXNS but set the link register",
so we leave it for a separate commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-21-git-send-email-peter.maydell@linaro.org

1 files changed, 13 insertions, 0 deletions

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 41e270ccdb..0f40a64206 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -419,7 +419,20 @@ typedef struct CPUARMState {
     } cp15;
 
     struct {
+        /* M profile has up to 4 stack pointers:
+         * a Main Stack Pointer and a Process Stack Pointer for each
+         * of the Secure and Non-Secure states. (If the CPU doesn't support
+         * the security extension then it has only two SPs.)
+         * In QEMU we always store the currently active SP in regs[13],
+         * and the non-active SP for the current security state in
+         * v7m.other_sp. The stack pointers for the inactive security state
+         * are stored in other_ss_msp and other_ss_psp.
+         * switch_v7m_security_state() is responsible for rearranging them
+         * when we change security state.
+         */
         uint32_t other_sp;
+        uint32_t other_ss_msp;
+        uint32_t other_ss_psp;
         uint32_t vecbase[2];
         uint32_t basepri[2];
         uint32_t control[2];