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

[Ingo Molnar]

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-and-bisected-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
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 fc878fee6a51..32826791e675 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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