Improve mstatus reconstruction

The instruction count in 'hal_context_save' has been reduced from 7 to
6.

Author: jserv

Documentation/hal-riscv-context-switch.md

@@ -48,20 +48,7 @@ typedef uint32_t jmp_buf[17];
 The `hal_context_save` function captures complete task state including both execution context and processor state.
 The function saves all callee-saved registers as required by the RISC-V ABI,
 plus essential pointers (gp, tp, sp, ra).
-For processor state, it performs sophisticated interrupt state reconstruction:
-
-```c
-/* mstatus reconstruction during timer interrupts */
-csrr t0, mstatus        // Read current mstatus (MIE=0 in trap)
-srli t1, t0, 4          // Shift MPIE (bit 7) to bit 3 position
-andi t1, t1, 8          // Isolate the reconstructed MIE bit
-li   t2, ~8             // Create mask to clear old MIE bit
-and  t0, t0, t2         // Clear the current MIE bit
-or   t0, t0, t1         // Set MIE to pre-trap value (from MPIE)
-sw   t0, 16*4(%0)       // Store in jmp_buf[16]
-```
-
-This ensures that tasks resume with correct interrupt state,
+For processor state, it performs sophisticated interrupt state reconstruction and ensures that tasks resume with correct interrupt state,
 maintaining system responsiveness and preventing interrupt state corruption.
 
 ### 2. Select Next Task

arch/riscv/hal.c

@@ -444,12 +444,11 @@ int32_t hal_context_save(jmp_buf env)
          * state from MPIE for consistent interrupt context preservation.
          */
         "csrr t0, mstatus\n" /* Read current mstatus (MIE=0 in trap) */
-        "srli t1, t0, 4\n"   /* Shift MPIE (bit 7) to bit 3 pos */
-        "andi t1, t1, 8\n"   /* Isolate the bit (MSTATUS_MIE) */
-        "li   t2, ~8\n"      /* Create mask to clear old MIE bit */
-        "and  t0, t0, t2\n"  /* Clear the current MIE bit */
-        "or   t0, t0, t1\n"  /* Set MIE to its pre-trap value (from MPIE) */
-        "sw   t0, 16*4(%0)\n"
+        "andi t1, t0, ~8\n"  /* Clear MIE bit first */
+        "srli t2, t0, 4\n"   /* Get MPIE bit to position 3 */
+        "andi t2, t2, 8\n"   /* Isolate bit 3 */
+        "or   t1, t1, t2\n"  /* Combine cleared MIE with reconstructed bit */
+        "sw   t1, 16*4(%0)\n"
         /* By convention, the initial call returns 0. */
         "li a0, 0\n"
         :