core: riscv: Fix logic of thread_{get/set}_exceptions()

In ARM, the bits in DAIF register are used to mask the interrupts. While
in RISC-V, the bits in CSR MIE/SIE are used to enable(unmask)
corresponding interrupt sources.

To not modify the function of thread_get_exceptions(), we invert the
bits after reading the value of CSR MIE/SIE, as mask.

To not modify the function of thread_set_exceptions(), we invert the
bits in given "exceptions" before writing "exceptions" into CSR
MIE/SIE. Therefore, the intended masked exception bits will be
cleared when we write the final value into CSR MIE/SIE to mask those
interrupts.

Signed-off-by: Alvin Chang <alvinga@andestech.com>

core/arch/riscv/kernel/thread_arch.c

@@ -32,9 +32,12 @@
 #include <trace.h>
 #include <util.h>
 
+/* This function returns current masked exception bits. */
 uint32_t __nostackcheck thread_get_exceptions(void)
 {
-	return read_csr(CSR_XIE) & THREAD_EXCP_ALL;
+	uint32_t xie = read_csr(CSR_XIE) & THREAD_EXCP_ALL;
+
+	return xie ^ THREAD_EXCP_ALL;
 }
 
 void __nostackcheck thread_set_exceptions(uint32_t exceptions)
@@ -43,6 +46,18 @@ void __nostackcheck thread_set_exceptions(uint32_t exceptions)
 	if (!(exceptions & THREAD_EXCP_FOREIGN_INTR))
 		assert_have_no_spinlock();
 
+	/*
+	 * In ARM, the bits in DAIF register are used to mask the exceptions.
+	 * While in RISC-V, the bits in CSR MIE/SIE are used to enable(unmask)
+	 * corresponding interrupt sources. To not modify the function of
+	 * thread_set_exceptions(), we should "invert" the bits in "exceptions".
+	 * The corresponding bits in "exceptions" will be inverted so they will
+	 * be cleared when we write the final value into CSR MIE/SIE. So that we
+	 * can mask those exceptions.
+	 */
+	exceptions &= THREAD_EXCP_ALL;
+	exceptions ^= THREAD_EXCP_ALL;
+
 	barrier();
 	write_csr(CSR_XIE, exceptions);
 	barrier();