1 files changed, 40 insertions, 5 deletions

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 19e4744b6eba..6530f1b8874d 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -1377,6 +1377,7 @@ void facility_unavailable_exception(struct pt_regs *regs)
 	};
 	char *facility = "unknown";
 	u64 value;
+	u32 instword, rd;
 	u8 status;
 	bool hv;
 
@@ -1388,12 +1389,46 @@ void facility_unavailable_exception(struct pt_regs *regs)
 
 	status = value >> 56;
 	if (status == FSCR_DSCR_LG) {
-		/* User is acessing the DSCR.  Set the inherit bit and allow
-		 * the user to set it directly in future by setting via the
-		 * FSCR DSCR bit.  We always leave HFSCR DSCR set.
+		/*
+		 * User is accessing the DSCR register using the problem
+		 * state only SPR number (0x03) either through a mfspr or
+		 * a mtspr instruction. If it is a write attempt through
+		 * a mtspr, then we set the inherit bit. This also allows
+		 * the user to write or read the register directly in the
+		 * future by setting via the FSCR DSCR bit. But in case it
+		 * is a read DSCR attempt through a mfspr instruction, we
+		 * just emulate the instruction instead. This code path will
+		 * always emulate all the mfspr instructions till the user
+		 * has attempted atleast one mtspr instruction. This way it
+		 * preserves the same behaviour when the user is accessing
+		 * the DSCR through privilege level only SPR number (0x11)
+		 * which is emulated through illegal instruction exception.
+		 * We always leave HFSCR DSCR set.
 		 */
-		current->thread.dscr_inherit = 1;
-		mtspr(SPRN_FSCR, value | FSCR_DSCR);
+		if (get_user(instword, (u32 __user *)(regs->nip))) {
+			pr_err("Failed to fetch the user instruction\n");
+			return;
+		}
+
+		/* Write into DSCR (mtspr 0x03, RS) */
+		if ((instword & PPC_INST_MTSPR_DSCR_USER_MASK)
+				== PPC_INST_MTSPR_DSCR_USER) {
+			rd = (instword >> 21) & 0x1f;
+			current->thread.dscr = regs->gpr[rd];
+			current->thread.dscr_inherit = 1;
+			mtspr(SPRN_FSCR, value | FSCR_DSCR);
+		}
+
+		/* Read from DSCR (mfspr RT, 0x03) */
+		if ((instword & PPC_INST_MFSPR_DSCR_USER_MASK)
+				== PPC_INST_MFSPR_DSCR_USER) {
+			if (emulate_instruction(regs)) {
+				pr_err("DSCR based mfspr emulation failed\n");
+				return;
+			}
+			regs->nip += 4;
+			emulate_single_step(regs);
+		}
 		return;
 	}