riscv: new TLS-based arch_is_user_context() implementation

This reverts the bulk of commit c8bfc2afda ("riscv: make
arch_is_user_context() SMP compatible") and replaces it with a flag
stored in the thread local storage (TLS) area, therefore making TLS
mandatory for userspace support on RISC-V.

This has many advantages:

- The tp (x4) register is already dedicated by the standard for this
  purpose, making TLS support almost free.

- This is very efficient, requiring only a single instruction to clear
  and 2 instructions to set.

- This makes the SMP case much more efficient. No need for funky
  exception code any longer.

- SMP and non-SMP now use the same implementation making maintenance
  easier.

- The is_user_mode variable no longer requires a dedicated PMP mapping
  and therefore freeing one PMP slot for other purposes.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

5f65dbcc9dab3d39473b05397e05.

arch/riscv/Kconfig

@@ -128,6 +128,7 @@ config RISCV_PMP
 	select SRAM_REGION_PERMISSIONS
 	select ARCH_MEM_DOMAIN_SYNCHRONOUS_API if USERSPACE
 	select ARCH_MEM_DOMAIN_DATA if USERSPACE
+	select THREAD_LOCAL_STORAGE if USERSPACE
 	help
 	  MCU implements Physical Memory Protection.
 

arch/riscv/core/isr.S

@@ -41,11 +41,9 @@
 	op fa6, __z_arch_esf_t_fa6_OFFSET(reg)	 ;\
 	op fa7, __z_arch_esf_t_fa7_OFFSET(reg)	 ;
 
-#define DO_CALLER_SAVED_T0T1(op) \
+#define DO_CALLER_SAVED(op) \
 	RV_E(	op t0, __z_arch_esf_t_t0_OFFSET(sp)	);\
-	RV_E(	op t1, __z_arch_esf_t_t1_OFFSET(sp)	)
-
-#define DO_CALLER_SAVED_REST(op) \
+	RV_E(	op t1, __z_arch_esf_t_t1_OFFSET(sp)	);\
 	RV_E(	op t2, __z_arch_esf_t_t2_OFFSET(sp)	);\
 	RV_I(	op t3, __z_arch_esf_t_t3_OFFSET(sp)	);\
 	RV_I(	op t4, __z_arch_esf_t_t4_OFFSET(sp)	);\
@@ -139,46 +137,12 @@ SECTION_FUNC(exception.entry, _isr_wrapper)
 
 	/* restore privileged stack pointer and zero the scratch reg */
 	csrrw sp, mscratch, sp
-
-#ifdef CONFIG_SMP
-	j 2f
-
-1:	/*
-	 * We were in user space. Determine if it attempted to execute an
-	 * arch_is_user_context() based on mscratch access. We want to return
-	 * to u-mode with t0!=0 as quickly as possible if so.
-	 */
-	addi sp, sp, -__z_arch_esf_t_SIZEOF
-	DO_CALLER_SAVED_T0T1(sr)		;
-	/* First, determine if we had an illegal instruction exception. */
-	csrr t0, mcause
-	li t1, SOC_MCAUSE_EXP_MASK
-	and t0, t0, t1
-	addi t0, t0, -2  /* = 2 = illegal instruction */
-	bnez t0, 3f
-	/* Make sure it was actually a "csrr t0, mscratch" */
-	csrr t0, mepc
-	lw t0, 0(t0)
-	li t1, 0x340022f3
-	bne t0, t1, 3f
-	/* So it was: skip over it and return leaving t0 clobbered. */
-	csrr t0, mepc
-	addi t0, t0, 4
-	csrw mepc, t0
-	lr t1, __z_arch_esf_t_t1_OFFSET(sp)
-	addi sp, sp, __z_arch_esf_t_SIZEOF
-	/* restore user stack pointer and leave */
-	csrrw sp, mscratch, sp
-	mret
-2:
-#endif /* CONFIG_SMP */
 1:
-#endif /* CONFIG_USERSPACE */
+#endif
 
 	/* Save caller-saved registers on current thread stack. */
 	addi sp, sp, -__z_arch_esf_t_SIZEOF
-	DO_CALLER_SAVED_T0T1(sr)		;
-3:	DO_CALLER_SAVED_REST(sr)		;
+	DO_CALLER_SAVED(sr)		;
 
 	/* Save s0 in the esf and load it with &_current_cpu. */
 	sr s0, __z_arch_esf_t_s0_OFFSET(sp)
@@ -199,17 +163,14 @@ SECTION_FUNC(exception.entry, _isr_wrapper)
 	/* save stack value to be restored later */
 	sr t0, __z_arch_esf_t_sp_OFFSET(sp)
 
-#if defined(CONFIG_THREAD_LOCAL_STORAGE)
 	/* Make sure tls pointer is sane */
 	lr t0, ___cpu_t_current_OFFSET(s0)
 	lr tp, _thread_offset_to_tls(t0)
-#endif
 
-#if !defined(CONFIG_SMP)
-	/* Clear user mode variable */
-	la t0, is_user_mode
-	sw zero, 0(t0)
-#endif
+	/* Clear our per-thread usermode flag */
+	lui t0, %tprel_hi(is_user_mode)
+	add t0, t0, tp, %tprel_add(is_user_mode)
+	sb zero, %tprel_lo(is_user_mode)(t0)
 #endif
 
 	/* Save MEPC register */
@@ -608,12 +569,11 @@ no_fp:	/* make sure this is reflected in the restored mstatus */
 	call z_riscv_pmp_usermode_enable
 #endif
 
-#if !defined(CONFIG_SMP)
-	/* Set user mode variable */
-	li t0, 1
-	la t1, is_user_mode
-	sw t0, 0(t1)
-#endif
+	/* Set our per-thread usermode flag */
+	li t1, 1
+	lui t0, %tprel_hi(is_user_mode)
+	add t0, t0, tp, %tprel_add(is_user_mode)
+	sb t1, %tprel_lo(is_user_mode)(t0)
 
 	/* load scratch reg with stack pointer for next exception entry */
 	add t0, sp, __z_arch_esf_t_SIZEOF
@@ -625,8 +585,7 @@ no_fp:	/* make sure this is reflected in the restored mstatus */
 	lr s0, __z_arch_esf_t_s0_OFFSET(sp)
 
 	/* Restore caller-saved registers from thread stack */
-	DO_CALLER_SAVED_T0T1(lr)
-	DO_CALLER_SAVED_REST(lr)
+	DO_CALLER_SAVED(lr)
 
 #ifdef CONFIG_USERSPACE
 	/* retrieve saved stack pointer */

arch/riscv/core/pmp.c

@@ -434,15 +434,6 @@ void z_riscv_pmp_usermode_prepare(struct k_thread *thread)
 	/* Retrieve pmpcfg0 partial content from global entries */
 	thread->arch.u_mode_pmpcfg_regs[0] = global_pmp_cfg[0];
 
-#if !defined(CONFIG_SMP)
-	/* Map the is_user_mode variable */
-	extern uint32_t is_user_mode;
-
-	set_pmp_entry(&index, PMP_R,
-		      (uintptr_t) &is_user_mode, sizeof(is_user_mode),
-		      PMP_U_MODE(thread));
-#endif
-
 	/* Map the usermode stack */
 	set_pmp_entry(&index, PMP_R | PMP_W,
 		      thread->stack_info.start, thread->stack_info.size,
@@ -542,11 +533,6 @@ int arch_mem_domain_max_partitions_get(void)
 	/* remove those slots dedicated to global entries */
 	available_pmp_slots -= global_pmp_end_index;
 
-#if !defined(CONFIG_SMP)
-	/* One slot needed to map the is_user_mode variable */
-	available_pmp_slots -= 1;
-#endif
-
 	/* At least one slot to map the user thread's stack */
 	available_pmp_slots -= 1;
 

arch/riscv/core/smp.c

@@ -32,6 +32,9 @@ void arch_start_cpu(int cpu_num, k_thread_stack_t *stack, int sz,
 
 void z_riscv_secondary_cpu_init(int cpu_num)
 {
+#ifdef CONFIG_THREAD_LOCAL_STORAGE
+	__asm__("mv tp, %0" : : "r" (z_idle_threads[cpu_num].tls));
+#endif
 #if defined(CONFIG_RISCV_SOC_INTERRUPT_INIT)
 	soc_interrupt_init();
 #endif
@@ -40,9 +43,6 @@ void z_riscv_secondary_cpu_init(int cpu_num)
 #endif
 #ifdef CONFIG_SMP
 	irq_enable(RISCV_MACHINE_SOFT_IRQ);
-#endif
-#ifdef CONFIG_THREAD_LOCAL_STORAGE
-	__asm__("mv tp, %0" : : "r" (z_idle_threads[cpu_num].tls));
 #endif
 	riscv_cpu_init[cpu_num].fn(riscv_cpu_init[cpu_num].arg);
 }

arch/riscv/core/thread.c

@@ -11,12 +11,11 @@
 #include <stdio.h>
 #include <pmp.h>
 
-#if defined(CONFIG_USERSPACE) && !defined(CONFIG_SMP)
+#ifdef CONFIG_USERSPACE
 /*
- * Glogal variable used to know the current mode running.
- * Is not boolean because it must match the PMP granularity of the arch.
+ * Per-thread (TLS) variable indicating whether execution is in user mode.
  */
-uint32_t is_user_mode;
+__thread uint8_t is_user_mode;
 #endif
 
 void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
@@ -246,9 +245,7 @@ FUNC_NORETURN void arch_user_mode_enter(k_thread_entry_t user_entry,
 	/* exception stack has to be in mscratch */
 	csr_write(mscratch, top_of_priv_stack);
 
-#if !defined(CONFIG_SMP)
 	is_user_mode = true;
-#endif
 
 	register void *a0 __asm__("a0") = user_entry;
 	register void *a1 __asm__("a1") = p1;

arch/riscv/include/kernel_arch_func.h

@@ -26,6 +26,9 @@ extern "C" {
 
 static ALWAYS_INLINE void arch_kernel_init(void)
 {
+#ifdef CONFIG_THREAD_LOCAL_STORAGE
+	__asm__ volatile ("li tp, 0");
+#endif
 #ifdef CONFIG_USERSPACE
 	csr_write(mscratch, 0);
 #endif

include/zephyr/arch/riscv/syscall.h

@@ -147,35 +147,19 @@ static inline uintptr_t arch_syscall_invoke0(uintptr_t call_id)
 }
 
 #ifdef CONFIG_USERSPACE
+register ulong_t riscv_tp_reg __asm__ ("tp");
+
 static inline bool arch_is_user_context(void)
 {
-#ifdef CONFIG_SMP
-	/*
-	 * This is painful. There is no way for u-mode code to know if we're
-	 * currently executing in u-mode without generating a fault, besides
-	 * stealing a general purpose register away from the standard ABI
-	 * that is. And a global variable doesn't work on SMP as this must be
-	 * per-CPU and we could be migrated to another CPU just at the right
-	 * moment to peek at the wrong CPU variable (and u-mode can't disable
-	 * preemption either).
-	 *
-	 * So, given that we'll have to pay the price of an exception entry
-	 * anyway, let's at least make it free to privileged threads by using
-	 * the mscratch register as the non-user context indicator (it must
-	 * be zero in m-mode for exception entry to work properly). In the
-	 * case of u-mode we'll simulate a proper return value in the
-	 * exception trap code. Let's settle on the return value in t0
-	 * and omit the volatile to give the compiler a chance to cache
-	 * the result.
-	 */
-	register ulong_t is_user __asm__ ("t1");
-	__asm__ ("csrr %0, mscratch" : "=r" (is_user));
-	return is_user != 0;
-#else
+	/* don't try accessing TLS variables if tp is not initialized */
+	if (riscv_tp_reg == 0) {
+		return false;
+	}
+
 	/* Defined in arch/riscv/core/thread.c */
-	extern uint32_t is_user_mode;
-	return is_user_mode;
-#endif
+	extern __thread uint8_t is_user_mode;
+
+	return is_user_mode != 0;
 }
 #endif