riscv: uaccess: Only restore the CSR_STATUS SUM bit

During switch to csrs will OR the value of the register into the
corresponding csr. In this case we're only interested in restoring the
SUM bit not the entire register.

Signed-off-by: Cyril Bur <cyrilbur@tenstorrent.com>
Link: https://lore.kernel.org/r/20250522160954.429333-1-cyrilbur@tenstorrent.com
Co-developed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Fixes: 788aa64 ("riscv: save the SR_SUM status over switches")
Link: https://lore.kernel.org/r/20250602121543.1544278-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

Author: cyrilbur-tenstorrent

arch/riscv/include/asm/processor.h

@@ -110,7 +110,7 @@ struct thread_struct {
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
 	unsigned long envcfg;
-	unsigned long status;
+	unsigned long sum;
 	u32 riscv_v_flags;
 	u32 vstate_ctrl;
 	struct __riscv_v_ext_state vstate;

arch/riscv/kernel/asm-offsets.c

@@ -34,7 +34,7 @@ void asm_offsets(void)
 	OFFSET(TASK_THREAD_S9, task_struct, thread.s[9]);
 	OFFSET(TASK_THREAD_S10, task_struct, thread.s[10]);
 	OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
-	OFFSET(TASK_THREAD_STATUS, task_struct, thread.status);
+	OFFSET(TASK_THREAD_SUM, task_struct, thread.sum);
 
 	OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
@@ -347,8 +347,8 @@ void asm_offsets(void)
 		  offsetof(struct task_struct, thread.s[11])
 		- offsetof(struct task_struct, thread.ra)
 	);
-	DEFINE(TASK_THREAD_STATUS_RA,
-		  offsetof(struct task_struct, thread.status)
+	DEFINE(TASK_THREAD_SUM_RA,
+		  offsetof(struct task_struct, thread.sum)
 		- offsetof(struct task_struct, thread.ra)
 	);
 

arch/riscv/kernel/entry.S

@@ -399,14 +399,15 @@ SYM_FUNC_START(__switch_to)
 	REG_S s11, TASK_THREAD_S11_RA(a3)
 
 	/* save the user space access flag */
-	li    s0, SR_SUM
-	csrr  s1, CSR_STATUS
-	REG_S s1, TASK_THREAD_STATUS_RA(a3)
+	csrr  s0, CSR_STATUS
+	REG_S s0, TASK_THREAD_SUM_RA(a3)
 
 	/* Save the kernel shadow call stack pointer */
 	scs_save_current
 	/* Restore context from next->thread */
-	REG_L s0,  TASK_THREAD_STATUS_RA(a4)
+	REG_L s0,  TASK_THREAD_SUM_RA(a4)
+	li    s1,  SR_SUM
+	and   s0,  s0, s1
 	csrs  CSR_STATUS, s0
 	REG_L ra,  TASK_THREAD_RA_RA(a4)
 	REG_L sp,  TASK_THREAD_SP_RA(a4)