arch: riscv: support CONFIG_USERSPACE in CONFIG_RISCV_CURRENT_VIA_GP

Reset the the `gp` register to `_kernel->cpus[i].current` when
`CONFIG_USERSPACE` is enabled on exception to keep it sane.

Updated the testcase to test both `CONFIG_RISCV_GP` and
`CONFIG_RISCV_CURRENT_VIA_GP`.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

arch/riscv/Kconfig

@@ -30,7 +30,7 @@ config RISCV_GP
 
 config RISCV_CURRENT_VIA_GP
 	bool "Store current thread into the global pointer (GP) register"
-	depends on !RISCV_GP && !USERSPACE
+	depends on !RISCV_GP
 	depends on MP_MAX_NUM_CPUS > 1
 	select ARCH_HAS_CUSTOM_CURRENT_IMPL
 	help

arch/riscv/core/isr.S

@@ -169,7 +169,9 @@ SECTION_FUNC(exception.entry, _isr_wrapper)
 	.option norelax
 	la gp, __global_pointer$
 	.option pop
-#endif /* CONFIG_RISCV_GP */
+#elif defined(CONFIG_RISCV_CURRENT_VIA_GP)
+	lr gp, ___cpu_t_current_OFFSET(s0)
+#endif /* CONFIG_RISCV_GP / CONFIG_RISCV_CURRENT_VIA_GP */
 
 	/* Clear our per-thread usermode flag */
 	lui t0, %tprel_hi(is_user_mode)

tests/arch/riscv/userspace/riscv_gp/prj.conf

@@ -1,3 +1,2 @@
 CONFIG_ZTEST=y
-CONFIG_RISCV_GP=y
 CONFIG_TEST_USERSPACE=y

tests/arch/riscv/userspace/riscv_gp/src/main.c

@@ -10,6 +10,10 @@
 #include <zephyr/kernel.h>
 #include <zephyr/ztest.h>
 
+#if !defined(CONFIG_RISCV_GP) && !defined(CONFIG_RISCV_CURRENT_VIA_GP)
+#error "CONFIG_RISCV_GP or CONFIG_RISCV_CURRENT_VIA_GP must be enabled for this test"
+#endif
+
 #define ROGUE_USER_STACK_SZ 2048
 
 static struct k_thread rogue_user_thread;
@@ -18,22 +22,58 @@ static K_THREAD_STACK_DEFINE(rogue_user_stack, ROGUE_USER_STACK_SZ);
 static void rogue_user_fn(void *p1, void *p2, void *p3)
 {
 	zassert_true(k_is_user_context());
+	uintptr_t gp_val = reg_read(gp);
+	uintptr_t gp_test_val;
 
+	/* Make sure that `gp` is as expected */
+	if (IS_ENABLED(CONFIG_RISCV_GP)) {
+		__asm__ volatile("la %0, __global_pointer$" : "=r" (gp_test_val));
+	} else { /* CONFIG_RISCV_CURRENT_VIA_GP */
+		gp_test_val = (uintptr_t)k_current_get();
+	}
+
+	/* Corrupt `gp` reg */
 	reg_write(gp, 0xbad);
-	zassert_equal(reg_read(gp), 0xbad);
+
+	/* Make sure that `gp` is corrupted */
+	if (IS_ENABLED(CONFIG_RISCV_GP)) {
+		zassert_equal(reg_read(gp), 0xbad);
+	} else { /* CONFIG_RISCV_CURRENT_VIA_GP */
+		zassert_equal((uintptr_t)arch_current_thread(), 0xbad);
+	}
+
+	/* Sleep to force a context switch, which will sanitize `gp` */
+	k_msleep(50);
+
+	/* Make sure that `gp` is sane again */
+	if (IS_ENABLED(CONFIG_RISCV_GP)) {
+		__asm__ volatile("la %0, __global_pointer$" : "=r" (gp_test_val));
+	} else { /* CONFIG_RISCV_CURRENT_VIA_GP */
+		gp_test_val = (uintptr_t)k_current_get();
+	}
+
+	zassert_equal(gp_val, gp_test_val);
 }
 
 ZTEST_USER(riscv_gp, test_gp_value)
 {
 	uintptr_t gp_val = reg_read(gp);
+	uintptr_t gp_test_val;
 	k_tid_t th;
 
-	zassert_not_equal(gp_val, 0);
+	if (IS_ENABLED(CONFIG_RISCV_GP)) {
+		__asm__ volatile("la %0, __global_pointer$" : "=r" (gp_test_val));
+	} else { /* CONFIG_RISCV_CURRENT_VIA_GP */
+		gp_test_val = (uintptr_t)k_current_get();
+	}
+	zassert_equal(gp_val, gp_test_val);
 
+	/* Create and run a rogue thread to corrupt the `gp` */
 	th = k_thread_create(&rogue_user_thread, rogue_user_stack, ROGUE_USER_STACK_SZ,
 			     rogue_user_fn, NULL, NULL, NULL, -1, K_USER, K_NO_WAIT);
 	zassert_ok(k_thread_join(th, K_FOREVER));
 
+	/* Make sure that `gp` is the same as before a rogue thread was executed */
 	zassert_equal(reg_read(gp), gp_val, "`gp` corrupted by user thread");
 }
 

tests/arch/riscv/userspace/riscv_gp/testcase.yaml

@@ -3,6 +3,13 @@ common:
   ignore_qemu_crash: true
   tags: kernel riscv
   platform_allow:
-    - qemu_riscv64
+    - qemu_riscv64/qemu_virt_riscv64/smp
 tests:
-  arch.riscv64.riscv_gp: {}
+  arch.riscv64.riscv_gp.relative_addressing:
+    extra_configs:
+      - CONFIG_RISCV_GP=y
+      - CONFIG_RISCV_CURRENT_VIA_GP=n
+  arch.riscv64.riscv_gp.thread_pointer:
+    extra_configs:
+      - CONFIG_RISCV_CURRENT_VIA_GP=y
+      - CONFIG_RISCV_GP=n