[PATCH] RISC-V: restore U-mode counter access (scounteren)

[Manfred SCHLAEGL]

In commit 5bc409786752 ("RISC-V: KVM: Remove scounteren initialization")
the initialization of scounteren was moved from the hypervisor (KVM) to
the supervisor -- a well-argued change. However, the same change also
restricted access to performance counters without comment: previously,
scounteren was set to 0x7 (0b111), allowing U-mode (userspace) access
to instret (IR), cycle (CY), and time (TM). After the change,
scounteren is set to 0x2 (0b010), which allows access only to time (TM)
and prohibits access to instret (IR) and cycle (CY). This breaks user-
space programs that directly use these performance counters.

This change restores the prior behavior: scounteren is set to 0x7,
which re-enables U-mode (userspace) access to instret (IR), cycle (CY),
and time (TM).

Fixes: 5bc409786752 ("RISC-V: KVM: Remove scounteren initialization")
Signed-off-by: Manfred SCHLAEGL <manfred.schlaegl@xxxxxx>
---
 arch/riscv/kernel/head.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 9c99c5ad6fe8..5ad5e12a8299 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -133,8 +133,8 @@ secondary_start_sbi:
 	csrw CSR_IP, zero
 
 #ifndef CONFIG_RISCV_M_MODE
-	/* Enable time CSR */
-	li t0, 0x2
+	/* Enable CY, TM, and IR counters in U mode */
+	li t0, 0x7
 	csrw CSR_SCOUNTEREN, t0
 #endif
 
@@ -247,8 +247,8 @@ SYM_CODE_START(_start_kernel)
 	 */
 	csrr a0, CSR_MHARTID
 #else
-	/* Enable time CSR */
-	li t0, 0x2
+	/* Enable CY, TM, and IR counters in U mode */
+	li t0, 0x7
 	csrw CSR_SCOUNTEREN, t0
 #endif /* CONFIG_RISCV_M_MODE */
 
-- 
2.47.3