Commit 1ea0d14e authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge Committed by Ingo Molnar
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x86/i386: Make sure stack-protector segment base is cache aligned

The Intel Optimization Reference Guide says:

	In Intel Atom microarchitecture, the address generation unit
	assumes that the segment base will be 0 by default. Non-zero
	segment base will cause load and store operations to experience
	a delay.
		- If the segment base isn't aligned to a cache line
		  boundary, the max throughput of memory operations is
		  reduced to one [e]very 9 cycles.
	Assembly/Compiler Coding Rule 15. (H impact, ML generality)
	For Intel Atom processors, use segments with base set to 0
	whenever possible; avoid non-zero segment base address that is
	not aligned to cache line boundary at all cost.

We can't avoid having a non-zero base for the stack-protector
segment, but we can make it cache-aligned.
Signed-off-by: default avatarJeremy Fitzhardinge <>
Cc: <>
LKML-Reference: <>
Signed-off-by: default avatarIngo Molnar <>
parent 23386d63
......@@ -403,7 +403,17 @@ extern unsigned long kernel_eflags;
extern asmlinkage void ignore_sysret(void);
#else /* X86_64 */
DECLARE_PER_CPU(unsigned long, stack_canary);
* Make sure stack canary segment base is cached-aligned:
* "For Intel Atom processors, avoid non zero segment base address
* that is not aligned to cache line boundary at all cost."
* (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
struct stack_canary {
char __pad[20]; /* canary at %gs:20 */
unsigned long canary;
DECLARE_PER_CPU(struct stack_canary, stack_canary) ____cacheline_aligned;
#endif /* X86_64 */
......@@ -78,14 +78,14 @@ static __always_inline void boot_init_stack_canary(void)
#ifdef CONFIG_X86_64
percpu_write(irq_stack_union.stack_canary, canary);
percpu_write(stack_canary, canary);
percpu_write(stack_canary.canary, canary);
static inline void setup_stack_canary_segment(int cpu)
#ifdef CONFIG_X86_32
unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu) - 20;
unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
struct desc_struct *gdt_table = get_cpu_gdt_table(cpu);
struct desc_struct desc;
......@@ -31,7 +31,7 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
"movl %P[task_canary](%[next]), %%ebx\n\t" \
"movl %%ebx, "__percpu_arg([stack_canary])"\n\t"
#define __switch_canary_oparam \
, [stack_canary] "=m" (per_cpu_var(stack_canary))
, [stack_canary] "=m" (per_cpu_var(stack_canary.canary))
#define __switch_canary_iparam \
, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
......@@ -1043,7 +1043,7 @@ DEFINE_PER_CPU(struct orig_ist, orig_ist);
#else /* CONFIG_X86_64 */
DEFINE_PER_CPU(unsigned long, stack_canary);
DEFINE_PER_CPU(struct stack_canary, stack_canary) ____cacheline_aligned;
/* Make sure %fs and %gs are initialized properly in idle threads */
......@@ -439,7 +439,6 @@ is386: movl $2,%ecx # set MP
jne 1f
movl $per_cpu__gdt_page,%eax
movl $per_cpu__stack_canary,%ecx
subl $20, %ecx
movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
shrl $16, %ecx
movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
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