[tip:x86/urgent] x86/fpu: Fix boot crash in the early FPU code

[tip-bot for Ingo Molnar]

Commit-ID:  b96fecbfa8c88b057e2bbf10021521c232bb3650
Gitweb:     http://git.kernel.org/tip/b96fecbfa8c88b057e2bbf10021521c232bb3650
Author:     Ingo Molnar <mingo@xxxxxxxxxx>
AuthorDate: Sat, 4 Jul 2015 09:58:19 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 4 Jul 2015 10:05:56 +0200

x86/fpu: Fix boot crash in the early FPU code

Jan Kara and Thomas Gleixner reported boot crashes in the FPU
code:

  general protection fault: 0000 [#1] SMP
  RIP: 0010:[<ffffffff81048a6c>]  [<ffffffff81048a6c>] mxcsr_feature_mask_init+0x1c/0x40

  2b:*  0f ae 85 00 fe ff ff    fxsave -0x200(%rbp)

and bisected it down to the following FPU commit:

   91a8c2a5b43f ("x86/fpu: Clean up and fix MXCSR handling")

The reason is that the on-stack FPU registers state variable,
used by the FXSAVE instruction, did not have the required
minimum alignment of 16 bytes, causing the general protection
fault.

This is most likely a GCC bug in older GCC versions, but the
offending commit also added a bogus extra 32-byte alignment
(which GCC ignored too).

So fix this bug by making the variable static again, but also
mark it __initdata this time, because fpu__init_system_mxcsr()
is now an __init function.

Reported-and-bisected-by: Jan Kara <jack@xxxxxxx>
Reported-bisected-and-tested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20150704075819.GA9201@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/fpu/init.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index fc878fe..3282679 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -95,11 +95,12 @@ static void __init fpu__init_system_mxcsr(void)
 	unsigned int mask = 0;
 
 	if (cpu_has_fxsr) {
-		struct fxregs_state fx_tmp __aligned(32) = { };
+		/* Static because GCC does not get 16-byte stack alignment right: */
+		static struct fxregs_state fxregs __initdata;
 
-		asm volatile("fxsave %0" : "+m" (fx_tmp));
+		asm volatile("fxsave %0" : "+m" (fxregs));
 
-		mask = fx_tmp.mxcsr_mask;
+		mask = fxregs.mxcsr_mask;
 
 		/*
 		 * If zero then use the default features mask,
--
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