KVM: LoongArch: selftests: Add system registers save/restore on exception

When system returns from exception with ertn instruction, PC comes from
LOONGARCH_CSR_ERA, and CSR.CRMD comes LOONGARCH_CSR_PRMD.

Here save CSR register CSR.ERA and CSR.PRMD into stack, and then restore
them from stack. So it can be modified by exception handlers in future.

Signed-off-by: Bibo Mao <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>

tools/testing/selftests/kvm/include/loongarch/processor.h

···
 	unsigned long pc;
 	unsigned long estat;
 	unsigned long badv;
+	unsigned long prmd;
 };
 
 #define PC_OFFSET_EXREGS		offsetof(struct ex_regs, pc)
 #define ESTAT_OFFSET_EXREGS		offsetof(struct ex_regs, estat)
 #define BADV_OFFSET_EXREGS		offsetof(struct ex_regs, badv)
+#define PRMD_OFFSET_EXREGS		offsetof(struct ex_regs, prmd)
 #define EXREGS_SIZE			sizeof(struct ex_regs)
 
 #else
 #define PC_OFFSET_EXREGS		((EXREGS_GPRS + 0) * 8)
 #define ESTAT_OFFSET_EXREGS		((EXREGS_GPRS + 1) * 8)
 #define BADV_OFFSET_EXREGS		((EXREGS_GPRS + 2) * 8)
-#define EXREGS_SIZE			((EXREGS_GPRS + 3) * 8)
+#define PRMD_OFFSET_EXREGS		((EXREGS_GPRS + 3) * 8)
+#define EXREGS_SIZE			((EXREGS_GPRS + 4) * 8)
 #endif
 
 #endif /* SELFTEST_KVM_PROCESSOR_H */

tools/testing/selftests/kvm/lib/loongarch/exception.S

···
 	st.d   t0, sp, ESTAT_OFFSET_EXREGS
 	csrrd  t0, LOONGARCH_CSR_BADV
 	st.d   t0, sp, BADV_OFFSET_EXREGS
+	csrrd  t0, LOONGARCH_CSR_PRMD
+	st.d   t0, sp, PRMD_OFFSET_EXREGS
 
 	or     a0, sp, zero
 	bl route_exception
+	ld.d   t0, sp, PC_OFFSET_EXREGS
+	csrwr  t0, LOONGARCH_CSR_ERA
+	ld.d   t0, sp, PRMD_OFFSET_EXREGS
+	csrwr  t0, LOONGARCH_CSR_PRMD
 	restore_gprs sp
 	csrrd  sp, LOONGARCH_CSR_KS0
 	ertn