s390: Static branches for machine features infrastructure

Provide infrastructure which allows to generate machine_has_<feature>()
functions, which are replacing the existing MACHINE_HAS_<feature> macros.
Such function usages generate a static branch depending on <feature>. The
static branch is patched using an alternative.

Each <feature> correlates with a bit set in the machine_features bit
field. If the corresponding bit is set, the branch will be patched. In
order to have any effect on branch patching feature bits must be set with
set_machine_features() in the decompressor before alternatives patching of
the kernel image.

It is possible to use clear_machine_feature() and test_machine_feature()
for machine features which cannot be completely detected within the
decompressor, e.g. if common code command line parameters allow to enable
or disable certain features. In such cases test_machine_feature() instead
of machine_has_feature() must be used within the kernel. This results in a
runtime check and not a static branch.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

3 files changed, 90 insertions, 2 deletions

diff --git a/arch/s390/include/asm/alternative.h b/arch/s390/include/asm/alternative.h
index 73e781b56bfe..67abef07ac93 100644
--- a/arch/s390/include/asm/alternative.h
+++ b/arch/s390/include/asm/alternative.h
@@ -32,8 +32,9 @@
 #define ALT_CTX_ALL		(ALT_CTX_EARLY | ALT_CTX_LATE)
 
 #define ALT_TYPE_FACILITY	0
-#define ALT_TYPE_SPEC		1
-#define ALT_TYPE_LOWCORE	2
+#define ALT_TYPE_FEATURE	1
+#define ALT_TYPE_SPEC		2
+#define ALT_TYPE_LOWCORE	3
 
 #define ALT_DATA_SHIFT		0
 #define ALT_TYPE_SHIFT		20
@@ -43,6 +44,10 @@
 					 ALT_TYPE_FACILITY << ALT_TYPE_SHIFT	| \
 					 (facility) << ALT_DATA_SHIFT)
 
+#define ALT_FEATURE(feature)		(ALT_CTX_EARLY << ALT_CTX_SHIFT		| \
+					 ALT_TYPE_FEATURE << ALT_TYPE_SHIFT	| \
+					 (feature) << ALT_DATA_SHIFT)
+
 #define ALT_SPEC(facility)		(ALT_CTX_LATE << ALT_CTX_SHIFT		| \
 					 ALT_TYPE_SPEC << ALT_TYPE_SHIFT	| \
 					 (facility) << ALT_DATA_SHIFT)
diff --git a/arch/s390/include/asm/machine.h b/arch/s390/include/asm/machine.h
new file mode 100644
index 000000000000..bbe6c24bfc88
--- /dev/null
+++ b/arch/s390/include/asm/machine.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright IBM Corp. 2024
+ */
+
+#ifndef __ASM_S390_MACHINE_H
+#define __ASM_S390_MACHINE_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/bitops.h>
+#include <asm/alternative.h>
+
+extern unsigned long machine_features[1];
+
+#define MAX_MFEATURE_BIT (sizeof(machine_features) * BITS_PER_BYTE)
+
+static inline void __set_machine_feature(unsigned int nr, unsigned long *mfeatures)
+{
+	if (nr >= MAX_MFEATURE_BIT)
+		return;
+	__set_bit(nr, mfeatures);
+}
+
+static inline void set_machine_feature(unsigned int nr)
+{
+	__set_machine_feature(nr, machine_features);
+}
+
+static inline void __clear_machine_feature(unsigned int nr, unsigned long *mfeatures)
+{
+	if (nr >= MAX_MFEATURE_BIT)
+		return;
+	__clear_bit(nr, mfeatures);
+}
+
+static inline void clear_machine_feature(unsigned int nr)
+{
+	__clear_machine_feature(nr, machine_features);
+}
+
+static bool __test_machine_feature(unsigned int nr, unsigned long *mfeatures)
+{
+	if (nr >= MAX_MFEATURE_BIT)
+		return false;
+	return test_bit(nr, mfeatures);
+}
+
+static bool test_machine_feature(unsigned int nr)
+{
+	return __test_machine_feature(nr, machine_features);
+}
+
+static __always_inline bool __test_machine_feature_constant(unsigned int nr)
+{
+	asm goto(
+		ALTERNATIVE("brcl 15,%l[l_no]", "brcl 0,0", ALT_FEATURE(%[nr]))
+		:
+		: [nr] "i" (nr)
+		:
+		: l_no);
+	return true;
+l_no:
+	return false;
+}
+
+#define DEFINE_MACHINE_HAS_FEATURE(name, feature)				\
+static __always_inline bool machine_has_##name(void)				\
+{										\
+	if (!__is_defined(__DECOMPRESSOR) && __builtin_constant_p(feature))	\
+		return __test_machine_feature_constant(feature);		\
+	return test_machine_feature(feature);					\
+}
+
+#endif /* __ASSEMBLY__ */
+#endif /* __ASM_S390_MACHINE_H */
diff --git a/arch/s390/kernel/alternative.c b/arch/s390/kernel/alternative.c
index 8d5d0de35de0..d3839fc0543d 100644
--- a/arch/s390/kernel/alternative.c
+++ b/arch/s390/kernel/alternative.c
@@ -5,6 +5,10 @@
 #include <asm/abs_lowcore.h>
 #include <asm/alternative.h>
 #include <asm/facility.h>
+#include <asm/sections.h>
+#include <asm/machine.h>
+
+unsigned long __bootdata_preserved(machine_features[1]);
 
 void __apply_alternatives(struct alt_instr *start, struct alt_instr *end, unsigned int ctx)
 {
@@ -23,6 +27,9 @@ void __apply_alternatives(struct alt_instr *start, struct alt_instr *end, unsign
 		case ALT_TYPE_FACILITY:
 			replace = test_facility(a->data);
 			break;
+		case ALT_TYPE_FEATURE:
+			replace = test_machine_feature(a->data);
+			break;
 		case ALT_TYPE_SPEC:
 			replace = nobp_enabled();
 			break;