[PATCH v1 2/2] riscv: envcfg save and restore on trap entry/exit

[Deepak Gupta <debug@rivosinc.com>]

envcfg CSR defines enabling bits for cache management instructions and soon
will control enabling for control flow integrity and pointer masking features.

Control flow integrity and pointer masking features need to be enabled on per
thread basis. Additionally, I believe cache management instructions need to be
enabled on per thread basis. As an example a seccomped task on riscv may be
restricted to not use cache management instructions

This patch creates a place holder for envcfg CSR in `thread_info` and adds
logic to save and restore on trap entry and exits. This allows such isa feature
to be enabled on per thread basis.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
 arch/riscv/include/asm/thread_info.h | 1 +
 arch/riscv/kernel/asm-offsets.c      | 1 +
 arch/riscv/kernel/entry.S            | 6 ++++++
 3 files changed, 8 insertions(+)

diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 574779900bfb..320bc899a63b 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -57,6 +57,7 @@ struct thread_info {
 	long			user_sp;	/* User stack pointer */
 	int			cpu;
 	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
+	unsigned long envcfg;
 #ifdef CONFIG_SHADOW_CALL_STACK
 	void			*scs_base;
 	void			*scs_sp;
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index a03129f40c46..cdd8f095c30c 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -39,6 +39,7 @@ void asm_offsets(void)
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
 	OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+	OFFSET(TASK_TI_ENVCFG, task_struct, thread_info.envcfg);
 #ifdef CONFIG_SHADOW_CALL_STACK
 	OFFSET(TASK_TI_SCS_SP, task_struct, thread_info.scs_sp);
 #endif
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 54ca4564a926..a1d87013f15a 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -64,12 +64,14 @@ SYM_CODE_START(handle_exception)
 	csrr s3, CSR_TVAL
 	csrr s4, CSR_CAUSE
 	csrr s5, CSR_SCRATCH
+	csrr s6, CSR_ENVCFG
 	REG_S s0, PT_SP(sp)
 	REG_S s1, PT_STATUS(sp)
 	REG_S s2, PT_EPC(sp)
 	REG_S s3, PT_BADADDR(sp)
 	REG_S s4, PT_CAUSE(sp)
 	REG_S s5, PT_TP(sp)
+	REG_S s6, TASK_TI_ENVCFG(tp)
 
 	/*
 	 * Set the scratch register to 0, so that if a recursive exception
@@ -129,6 +131,10 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	addi s0, sp, PT_SIZE_ON_STACK
 	REG_S s0, TASK_TI_KERNEL_SP(tp)
 
+	/* restore envcfg bits for current thread */
+	REG_L s0, TASK_TI_ENVCFG(tp)
+	csrw CSR_ENVCFG, s0
+
 	/* Save the kernel shadow call stack pointer */
 	scs_save_current
 
-- 
2.43.0