linux-kernel - [PATCH 1/6] riscv: Introduce use_alternative

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Message-Id: <20250820-riscv-altn-helper-wip-v1-1-c3c626c1f7e6@iscas.ac.cn>
Date: Wed, 20 Aug 2025 21:44:45 +0800
From: Vivian Wang <wangruikang@...as.ac.cn>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alex@...ti.fr>, Yury Norov <yury.norov@...il.com>, 
 Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Vivian Wang <wangruikang@...as.ac.cn>, Vivian Wang <uwu@...m.page>, 
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Aydın Mercan <aydin@...can.dev>
Subject: [PATCH 1/6] riscv: Introduce use_alternative_{likely,unlikely}

Introduce convenience helpers use_alternative_likely() and
use_alternative_unlikely() to implement the pattern of using asm goto to
check if an alternative is selected. Existing code will be converted in
subsequent patches.

Similar to arm64 alternative_has_cap_{likely,unlikely}, but for riscv,
alternatives are not all CPU capabilities.

Suggested-by: Aydın Mercan <aydin@...can.dev>
Signed-off-by: Vivian Wang <wangruikang@...as.ac.cn>
---
 arch/riscv/include/asm/alternative-macros.h | 73 +++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/arch/riscv/include/asm/alternative-macros.h b/arch/riscv/include/asm/alternative-macros.h
index 231d777d936c2d29c858decaa9a3fa5f172efbb8..be9835b5e4eba03d76db3a73da19ac9e2981c4db 100644
--- a/arch/riscv/include/asm/alternative-macros.h
+++ b/arch/riscv/include/asm/alternative-macros.h
@@ -158,4 +158,77 @@
 	_ALTERNATIVE_CFG_2(old_content, new_content_1, vendor_id_1, patch_id_1, CONFIG_k_1,	\
 					new_content_2, vendor_id_2, patch_id_2, CONFIG_k_2)
 
+/*
+ * use_alternative_{likely,unlikely}() returns true if the alternative is
+ * applied and false otherwise, but in a way where the compiler can optimize
+ * this check down to a nop instruction that's patched into a jump, or vice
+ * versa.
+ *
+ * Always returns false if the alternatives mechanism is not available.
+ *
+ * Usage example:
+ *   if (use_alternative_likely(0, RISCV_ISA_ZBB))
+ *
+ * Similar to static keys, "likely" means use a nop if the alternative is
+ * selected, and jump if unselected; "unlikely" is the other way around.
+ */
+
+#ifndef __ASSEMBLER__
+
+#include <linux/types.h>
+
+#ifdef CONFIG_RISCV_ALTERNATIVE
+
+static __always_inline bool use_alternative_likely(u16 vendor_id, u32 patch_id)
+{
+	BUILD_BUG_ON(!__builtin_constant_p(vendor_id));
+	BUILD_BUG_ON(!__builtin_constant_p(patch_id));
+
+	asm goto(ALTERNATIVE("j %l[no_alt]", "nop", %[vendor_id], %[patch_id], 1)
+		 :
+		 : [vendor_id] "i"(vendor_id),
+		   [patch_id] "i"(patch_id)
+		 :
+		 : no_alt);
+
+	return true;
+
+no_alt:
+	return false;
+}
+
+static __always_inline bool use_alternative_unlikely(u16 vendor_id, u32 patch_id)
+{
+	BUILD_BUG_ON(!__builtin_constant_p(vendor_id));
+	BUILD_BUG_ON(!__builtin_constant_p(patch_id));
+
+	asm goto(ALTERNATIVE("nop", "j %l[alt]", %[vendor_id], %[patch_id], 1)
+		 :
+		 : [vendor_id] "i"(vendor_id),
+		   [patch_id] "i"(patch_id)
+		 :
+		 : alt);
+
+	return false;
+
+alt:
+	return true;
+}
+
+#else
+
+static inline bool use_alternative_likely(u16 vendor_id, u32 patch_id)
+{
+	return false;
+}
+
+static inline bool use_alternative_unlikely(u16 vendor_id, u32 patch_id)
+{
+	return false;
+}
+
+#endif /* CONFIG_RISCV_ALTERNATIVE */
+
+#endif /* __ASSEMBLER__ */
+
 #endif

-- 
2.50.1