fix(riscv/irqc): align IMSIC handler to AIA specifications

"The RISC-V Advanced Interrupt Architecture" specifications explain
in Chapter 3 (pages 26-27) how interrupts should be handled with IMSIC,
in particular, interrupt claiming has to be done right after reading the
STOPEI register or in the same read instruction with the csrrw
instruction.

This commit aligns the IMSIC interrupt handler to that specs.
Without this change, if an interrupt with higher priority becomes
pending while handling a lower one, writing to STOPEI after
`interrupts_handle()` executes will claim the higher IRQ, freezing
the execution.

Signed-off-by: Filippo Fontana <filippo.fontana@minervasys.tech>

Author: filippofontana

src/arch/riscv/inc/arch/csrs.h

@@ -246,6 +246,14 @@
     static inline void csrs_##csr_name##_clear(unsigned long csr_value)           \
     {                                                                             \
         __asm__ volatile("csrc " XSTR(csr_id) ",%0" ::"r"(csr_value) : "memory"); \
+    }                                                                             \
+    static inline unsigned long csrs_##csr_name##_swap(unsigned long csr_value)   \
+    {                                                                             \
+        unsigned long reg_val;                                                    \
+        __asm__ volatile("csrrw %0," XSTR(csr_id) ",%1" : "=r"(reg_val)           \
+                                                        : "rK"(csr_value)         \
+                                                        : "memory");              \
+        return reg_val;                                                           \
     }
 
 #define CSRS_GEN_ACCESSORS(csr) CSRS_GEN_ACCESSORS_NAMED(csr, csr)

src/arch/riscv/irqc/aia/imsic.c

@@ -87,14 +87,11 @@ void imsic_send_msi(cpuid_t target_cpu, irqid_t ipi_id)
 
 void imsic_handle(void)
 {
-    uint32_t intp_identity = (uint32_t)(csrs_stopei_read() >> STOPEI_EEID);
+    /* Read STOPEI and write to it to claim the interrupt */
+    uint32_t intp_identity = (uint32_t)(csrs_stopei_swap(0) >> STOPEI_EEID);
 
     if (intp_identity != 0) {
-        enum irq_res res = interrupts_handle(intp_identity);
-        if (res == HANDLED_BY_HYP) {
-            /* Write to STOPEI to clear the interrupt */
-            csrs_stopei_write(0);
-        }
+        interrupts_handle(intp_identity);
     };
 }