From b8c1b8ea7b219a7ba6d58d97bfdf1403b741f8d5 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Sun, 24 May 2015 09:58:12 +0200 Subject: [PATCH] x86/fpu: Fix FPU state save area alignment bug On most configs task-struct is cache line aligned, which makes the XSAVE area's 64-byte required alignment work out fine. But on some .config's task_struct is aligned only to 16 bytes (enforced by ARCH_MIN_TASKALIGN), which makes things like fpu__copy() (that XSAVEOPT uses) not work so well. I broke this in: 7366ed771f6e ("x86/fpu: Simplify FPU handling by embedding the fpstate in task_struct (again)") which embedded the fpstate in the task_struct. The alignment requirements of the FPU code were originally present in ARCH_MIN_TASKALIGN, which still has a value of 16, which was the alignment requirement of the FPU state area prior XSAVE. But this link was not documented (and not required) and the link got lost when the FPU state area was made dynamic years ago. With XSAVEOPT the minimum alignment requirment went up to 64 bytes, and the embedding of the FPU state area in task_struct exposed it again - and '16' was not increased to '64'. So fix this bug, but also try to address the underlying lost link of information that made it easier to happen: - document ARCH_MIN_TASKALIGN a bit better - use alignof() to recover the current alignment requirements. This would work in the future as well, should the alignment requirements go up to 128 bytes with things like AVX512. ( We should probably also use the vSMP alignment rules for all of x86, but that's for another patch. ) Reported-by: Peter Zijlstra Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Dave Hansen Cc: Fenghua Yu Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Thomas Gleixner Signed-off-by: Ingo Molnar --- arch/x86/include/asm/processor.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index b9e487499ae2..8e04f51d6bea 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -53,11 +53,16 @@ static inline void *current_text_addr(void) return pc; } +/* + * These alignment constraints are for performance in the vSMP case, + * but in the task_struct case we must also meet hardware imposed + * alignment requirements of the FPU state: + */ #ifdef CONFIG_X86_VSMP # define ARCH_MIN_TASKALIGN (1 << INTERNODE_CACHE_SHIFT) # define ARCH_MIN_MMSTRUCT_ALIGN (1 << INTERNODE_CACHE_SHIFT) #else -# define ARCH_MIN_TASKALIGN 16 +# define ARCH_MIN_TASKALIGN __alignof__(union fpregs_state) # define ARCH_MIN_MMSTRUCT_ALIGN 0 #endif -- 2.20.1