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Subject[tip:x86/urgent] x86/fpu: Fix boot crash in the early FPU code
Commit-ID:  b96fecbfa8c88b057e2bbf10021521c232bb3650
Gitweb: http://git.kernel.org/tip/b96fecbfa8c88b057e2bbf10021521c232bb3650
Author: Ingo Molnar <mingo@kernel.org>
AuthorDate: Sat, 4 Jul 2015 09:58:19 +0200
Committer: Ingo Molnar <mingo@kernel.org>
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@suse.cz>
Reported-bisected-and-tested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150704075819.GA9201@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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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