[PATCH v4 2/3] riscv: Add support for per-thread envcfg CSR values

[Samuel Holland]

Some bits in the [ms]envcfg CSR, such as the CFI state and pointer
masking mode, need to be controlled on a per-thread basis. Support this
by keeping a copy of the CSR value in struct thread_struct and writing
it during context switches. It is safe to discard the old CSR value
during the context switch because the CSR is modified only by software,
so the CSR will remain in sync with the copy in thread_struct.

Use ALTERNATIVE directly instead of riscv_has_extension_unlikely() to
minimize branchiness in the context switching code.

Since thread_struct is copied during fork(), setting the value for the
init task sets the default value for all other threads.

Reviewed-by: Andrew Jones <ajones at ventanamicro.com>
Reviewed-by: Deepak Gupta <debug at rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

(no changes since v1)

 arch/riscv/include/asm/processor.h | 1 +
 arch/riscv/include/asm/switch_to.h | 8 ++++++++
 arch/riscv/kernel/cpufeature.c     | 2 +-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 8702b8721a27..586e4ab701c4 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -124,6 +124,7 @@ struct thread_struct {
 	unsigned long s[12];	/* s[0]: frame pointer */
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
+	unsigned long envcfg;
 	u32 riscv_v_flags;
 	u32 vstate_ctrl;
 	struct __riscv_v_ext_state vstate;
diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 7594df37cc9f..9685cd85e57c 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -70,6 +70,13 @@ static __always_inline bool has_fpu(void) { return false; }
 #define __switch_to_fpu(__prev, __next) do { } while (0)
 #endif
 
+static inline void __switch_to_envcfg(struct task_struct *next)
+{
+	asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0",
+				  0, RISCV_ISA_EXT_XLINUXENVCFG, 1)
+			:: "r" (next->thread.envcfg) : "memory");
+}
+
 extern struct task_struct *__switch_to(struct task_struct *,
 				       struct task_struct *);
 
@@ -103,6 +110,7 @@ do {							\
 		__switch_to_vector(__prev, __next);	\
 	if (switch_to_should_flush_icache(__next))	\
 		local_flush_icache_all();		\
+	__switch_to_envcfg(__next);			\
 	((last) = __switch_to(__prev, __next));		\
 } while (0)
 
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 0139d4ea8426..df3e7e8d6d78 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -922,7 +922,7 @@ unsigned long riscv_get_elf_hwcap(void)
 void riscv_user_isa_enable(void)
 {
 	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
-		csr_set(CSR_ENVCFG, ENVCFG_CBZE);
+		current->thread.envcfg |= ENVCFG_CBZE;
 	else if (any_cpu_has_zicboz)
 		pr_warn_once("Zicboz disabled as it is unavailable on some harts\n");
 }
-- 
2.45.1