[RFC PATCH] riscv: cpu: check U-Mode before counteren write

[Nikita Shubin]

From: Nikita Shubin <n.shubin at yadro.com>

The Priv ISA states:
"In systems without U-mode, the mcounteren register should
not exist."

Check U-Mode is present in MISA before writing to counteren, otherwise
we endup with Illegal Instruction exception on systems without U-Mode.

Also make checking MISA default for M-Mode.

Signed-off-by: Nikita Shubin <n.shubin at yadro.com>
---
This seems obvious at first glance, but i've never seen 'u' extension 
enywhere in "riscv,isa" device tree property, even qemu doesn't set this, 
and if we simply enable this check - this will break existing board for sure.

We can rely on MISA completely if we are in M-Mode, as we currently check only
'd', 'f' and 'u', which are standart and nothing fancy.
---
 arch/riscv/cpu/cpu.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
index 0f323b26b3..917f71c8d7 100644
--- a/arch/riscv/cpu/cpu.c
+++ b/arch/riscv/cpu/cpu.c
@@ -31,7 +31,9 @@ u32 available_harts_lock = 1;
 
 static inline bool supports_extension(char ext)
 {
-#ifdef CONFIG_CPU
+#if CONFIG_IS_ENABLED(RISCV_MMODE)
+	return csr_read(CSR_MISA) & (1 << (ext - 'a'));
+#elif CONFIG_CPU
 	struct udevice *dev;
 	char desc[32];
 
@@ -48,13 +50,9 @@ static inline bool supports_extension(char ext)
 
 	return false;
 #else  /* !CONFIG_CPU */
-#if CONFIG_IS_ENABLED(RISCV_MMODE)
-	return csr_read(CSR_MISA) & (1 << (ext - 'a'));
-#else  /* !CONFIG_IS_ENABLED(RISCV_MMODE) */
 #warning "There is no way to determine the available extensions in S-mode."
 #warning "Please convert your board to use the RISC-V CPU driver."
 	return false;
-#endif /* CONFIG_IS_ENABLED(RISCV_MMODE) */
 #endif /* CONFIG_CPU */
 }
 
@@ -102,12 +100,14 @@ int riscv_cpu_setup(void *ctx, struct event *event)
 		 * Enable perf counters for cycle, time,
 		 * and instret counters only
 		 */
+		if (supports_extension('u')) {
 #ifdef CONFIG_RISCV_PRIV_1_9
-		csr_write(CSR_MSCOUNTEREN, GENMASK(2, 0));
-		csr_write(CSR_MUCOUNTEREN, GENMASK(2, 0));
+			csr_write(CSR_MSCOUNTEREN, GENMASK(2, 0));
+			csr_write(CSR_MUCOUNTEREN, GENMASK(2, 0));
 #else
-		csr_write(CSR_MCOUNTEREN, GENMASK(2, 0));
+			csr_write(CSR_MCOUNTEREN, GENMASK(2, 0));
 #endif
+		}
 
 		/* Disable paging */
 		if (supports_extension('s'))
-- 
2.37.4