Re: [PATCH] ARC: ARCv2: jump label: implement jump label patching

Peter Zijlstra
2019-06-21T12:09:23Z

On Thu, Jun 20, 2019 at 11:22:56PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 20, 2019 at 11:48:17AM -0700, Vineet Gupta wrote:

> > I do worry about the occasional alignment induced extra NOP_S instruction (2 byte)
> > but there doesn't seem to be an easy solution. Heck if we could use the NOP_S /
> > B_S in first place. While not a clean solution by any standards, could anything be
> > done to reduce the code path of DO_ONCE() so that unlikely code is not too far off.
>
> if one could somehow get the arch_static_branch*() things to
> conditionally emit either the 2 or 4 byte jump, depending on the offset
> (which is known there, since we stick it in the __jump_table), then we
> can have arch_jump_label_transform() use that same condition to switch
> between 2 and 4 bytes too.
>
> I just don't know if it's possible :-/

So I had to try; but GAS macro .if directives don't like labels as
arguments, not constant enough for them.

../arch/x86/include/asm/jump_label.h:26: Error: non-constant expression in ".if" statement

Damn!

---
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -12,24 +12,19 @@
# define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC
#endif

-#include <asm/asm.h>
-#include <asm/nops.h>
+asm(".include \"asm/jump_label_asm.h\"");

#ifndef __ASSEMBLY__

#include <linux/stringify.h>
#include <linux/types.h>
+#include <asm/asm.h>
+#include <asm/nops.h>

static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
- asm_volatile_goto("1:"
- ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
- ".pushsection __jump_table, \"aw\" \n\t"
- _ASM_ALIGN "\n\t"
- ".long 1b - ., %l[l_yes] - . \n\t"
- _ASM_PTR "%c0 + %c1 - .\n\t"
- ".popsection \n\t"
- : : "i" (key), "i" (branch) : : l_yes);
+ asm_volatile_goto("STATIC_BRANCH_NOP l_yes=\"%l[l_yes]\", key=\"%c0\", branch=\"%c1\""
+ : : "i" (key), "i" (branch) : : l_yes);

return false;
l_yes:
@@ -38,57 +33,13 @@ static __always_inline bool arch_static_

static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
{
- asm_volatile_goto("1:"
- ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
- "2:\n\t"
- ".pushsection __jump_table, \"aw\" \n\t"
- _ASM_ALIGN "\n\t"
- ".long 1b - ., %l[l_yes] - . \n\t"
- _ASM_PTR "%c0 + %c1 - .\n\t"
- ".popsection \n\t"
- : : "i" (key), "i" (branch) : : l_yes);
+ asm_volatile_goto("STATIC_BRANCH_JMP l_yes=\"%l[l_yes]\", key=\"%c0\", branch=\"%c1\""
+ : : "i" (key), "i" (branch) : : l_yes);

return false;
l_yes:
return true;
}

-#else /* __ASSEMBLY__ */
-
-.macro STATIC_JUMP_IF_TRUE target, key, def
-.Lstatic_jump_\@:
- .if \def
- /* Equivalent to "jmp.d32 \target" */
- .byte 0xe9
- .long \target - .Lstatic_jump_after_\@
-.Lstatic_jump_after_\@:
- .else
- .byte STATIC_KEY_INIT_NOP
- .endif
- .pushsection __jump_table, "aw"
- _ASM_ALIGN
- .long .Lstatic_jump_\@ - ., \target - .
- _ASM_PTR \key - .
- .popsection
-.endm
-
-.macro STATIC_JUMP_IF_FALSE target, key, def
-.Lstatic_jump_\@:
- .if \def
- .byte STATIC_KEY_INIT_NOP
- .else
- /* Equivalent to "jmp.d32 \target" */
- .byte 0xe9
- .long \target - .Lstatic_jump_after_\@
-.Lstatic_jump_after_\@:
- .endif
- .pushsection __jump_table, "aw"
- _ASM_ALIGN
- .long .Lstatic_jump_\@ - ., \target - .
- _ASM_PTR \key + 1 - .
- .popsection
-.endm
-
-#endif /* __ASSEMBLY__ */
-
-#endif
+#endif /* __ASSEMBLY__ */
+#endif /* _ASM_X86_JUMP_LABEL_H */
--- /dev/null
+++ b/arch/x86/include/asm/jump_label_asm.h
@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_JUMP_LABEL_ASM_H
+#define _ASM_X86_JUMP_LABEL_ASM_H
+
+#include <asm/asm.h>
+#include <asm/nops.h>
+
+#ifdef __ASSEMBLY__
+
+.macro STATIC_BRANCH_ENTRY l_target:req l_yes:req key:req branch:req
+ .pushsection __jump_table, "aw"
+ .long \l_target - ., \l_yes - .
+#ifdef __X86_64__
+ .quad (\key - .) + \branch
+#else
+ .long (\key - .) + \branch
+#endif
+ .popsection
+.endm
+
+.macro STATIC_BRANCH_NOP l_yes:req key:req branch:req
+.Lstatic_branch_nop_\@:
+.iflt 127 - .
+ .byte 0x66, 0x90
+.else
+ .byte STATIC_KEY_INIT_NOP
+.endif
+ STATIC_BRANCH_ENTRY l_target=.Lstatic_branch_nop_\@, l_yes=\l_yes, key=\key, branch=\branch
+.endm
+
+.macro STATIC_BRANCH_JMP l_yes:req key:req branch:req
+.Lstatic_branch_jmp_\@:
+.if \l_yes - . < 127
+ .byte 0xeb
+ .byte \l_yes - (. + 1)
+.else
+ .byte 0xe9
+ .long \l_yes - (. + 4)
+.endif
+ STATIC_BRANCH_ENTRY l_target=.Lstatic_branch_jmp_\@, l_yes=\l_yes, key=\key, branch=\branch
+.endm
+
+#endif /* __ASSEMBLY__ */
+#endif /* _ASM_X86_JUMP_LABEL_ASM_H */