[RFC PATCH 1/8] static_call/x86: Add __static_call_return0()

[Frederic Weisbecker]

From: Peter Zijlstra <pet...@infradead.org>

Provide a stub function that return 0 and wire up the static call site
patching to replace the CALL with a single 5 byte instruction that
clears %RAX, the return value register.

The function can be cast to any function pointer type that has a
single %RAX return (including pointers). Also provide a version that
returns an int for convenience. We are clearing the entire %RAX register
in any case, whether the return value is 32 or 64 bits, since %RAX is
always a scratch register anyway.

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Mel Gorman <mgor...@suse.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Michal Hocko <mho...@kernel.org>
Cc: Paul E. McKenney <paul...@kernel.org>
Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
---
 arch/x86/kernel/static_call.c | 17 +++++++++++++++--
 include/linux/static_call.h   |  2 ++
 kernel/static_call.c          |  5 +++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index ca9a380d9c0b..9442c4136c38 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -11,14 +11,26 @@ enum insn_type {
        RET = 3,  /* tramp / site cond-tail-call */
 };
 
+/*
+ * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax
+ * The REX.W cancels the effect of any data16.
+ */
+static const u8 xor5rax[] = { 0x66, 0x66, 0x48, 0x31, 0xc0 };
+
 static void __ref __static_call_transform(void *insn, enum insn_type type, 
void *func)
 {
+       const void *emulate = NULL;
        int size = CALL_INSN_SIZE;
        const void *code;
 
        switch (type) {
        case CALL:
                code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
+               if (func == &__static_call_return0) {
+                       emulate = code;
+                       code = &xor5rax;
+               }
+
                break;
 
        case NOP:
@@ -41,7 +53,7 @@ static void __ref __static_call_transform(void *insn, enum 
insn_type type, void
        if (unlikely(system_state == SYSTEM_BOOTING))
                return text_poke_early(insn, code, size);
 
-       text_poke_bp(insn, code, size, NULL);
+       text_poke_bp(insn, code, size, emulate);
 }
 
 static void __static_call_validate(void *insn, bool tail)
@@ -54,7 +66,8 @@ static void __static_call_validate(void *insn, bool tail)
                        return;
        } else {
                if (opcode == CALL_INSN_OPCODE ||
-                   !memcmp(insn, ideal_nops[NOP_ATOMIC5], 5))
+                   !memcmp(insn, ideal_nops[NOP_ATOMIC5], 5) ||
+                   !memcmp(insn, xor5rax, 5))
                        return;
        }
 
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 695da4c9b338..9f05d60aca70 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -134,6 +134,8 @@ extern void arch_static_call_transform(void *site, void 
*tramp, void *func, bool
                             STATIC_CALL_TRAMP_ADDR(name), func);       \
 })
 
+extern long __static_call_return0(void);
+
 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
 
 extern int __init static_call_init(void);
diff --git a/kernel/static_call.c b/kernel/static_call.c
index 84565c2a41b8..0bc11b5ce681 100644
--- a/kernel/static_call.c
+++ b/kernel/static_call.c
@@ -438,6 +438,11 @@ int __init static_call_init(void)
 }
 early_initcall(static_call_init);
 
+long __static_call_return0(void)
+{
+       return 0;
+}
+
 #ifdef CONFIG_STATIC_CALL_SELFTEST
 
 static int func_a(int x)
-- 
2.25.1