x86/asm/entry/irq: Simplify interrupt dispatch table (IDT) layout

Interrupt entry points are handled with the following code,
each 32-byte code block contains seven entry points:

		...
		[push][jump 22] // 4 bytes
		[push][jump 18] // 4 bytes
		[push][jump 14] // 4 bytes
		[push][jump 10] // 4 bytes
		[push][jump  6] // 4 bytes
		[push][jump  2] // 4 bytes
		[push][jump common_interrupt][padding] // 8 bytes

		[push][jump]
		[push][jump]
		[push][jump]
		[push][jump]
		[push][jump]
		[push][jump]
		[push][jump common_interrupt][padding]

		[padding_2]
	common_interrupt:

And there is a table which holds pointers to every entry point,
IOW: to every push.

In cold cache, two jumps are still costlier than one, even
though we get the benefit of them residing in the same
cacheline.

This change replaces short jumps with near ones to
'common_interrupt', and pads every push+jump pair to 8 bytes. This
way, each interrupt takes only one jump.

This change replaces ".p2align CONFIG_X86_L1_CACHE_SHIFT" before
dispatch table with ".align 8" - we do not need anything
stronger than that.

The table of entry addresses (the interrupt[] array) is no
longer necessary, the address of entries can be easily
calculated as (irq_entries_start + i*8).

   text	   data	    bss	    dec	    hex	filename
  12546	      0	      0	  12546	   3102	entry_64.o.before
  11626	      0	      0	  11626	   2d6a	entry_64.o

The size decrease is because 1656 bytes of .init.rodata are
gone. That's initdata, though. The resident size does go up a
bit.

Run-tested (32 and 64 bits).

Acked-and-Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1428090553-7283-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 10 insertions, 31 deletions

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index effa2793feba..02bec0f1d1e1 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -723,43 +723,22 @@ END(sysenter_badsys)
 .endm
 
 /*
- * Build the entry stubs and pointer table with some assembler magic.
- * We pack 7 stubs into a single 32-byte chunk, which will fit in a
- * single cache line on all modern x86 implementations.
+ * Build the entry stubs with some assembler magic.
+ * We pack 1 stub into every 8-byte block.
  */
-.section .init.rodata,"a"
-ENTRY(interrupt)
-.section .entry.text, "ax"
-	.p2align 5
-	.p2align CONFIG_X86_L1_CACHE_SHIFT
+	.align 8
 ENTRY(irq_entries_start)
 	RING0_INT_FRAME
-vector=FIRST_EXTERNAL_VECTOR
-.rept (FIRST_SYSTEM_VECTOR-FIRST_EXTERNAL_VECTOR+6)/7
-	.balign 32
-  .rept	7
-    .if vector < FIRST_SYSTEM_VECTOR
-      .if vector <> FIRST_EXTERNAL_VECTOR
+    vector=FIRST_EXTERNAL_VECTOR
+    .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
+	pushl_cfi $(~vector+0x80)	/* Note: always in signed byte range */
+    vector=vector+1
+	jmp	common_interrupt
 	CFI_ADJUST_CFA_OFFSET -4
-      .endif
-1:	pushl_cfi $(~vector+0x80)	/* Note: always in signed byte range */
-      .if ((vector-FIRST_EXTERNAL_VECTOR)%7) <> 6
-	jmp 2f
-      .endif
-      .previous
-	.long 1b
-      .section .entry.text, "ax"
-vector=vector+1
-    .endif
-  .endr
-2:	jmp common_interrupt
-.endr
+	.align	8
+    .endr
 END(irq_entries_start)
 
-.previous
-END(interrupt)
-.previous
-
 /*
  * the CPU automatically disables interrupts when executing an IRQ vector,
  * so IRQ-flags tracing has to follow that: