[PATCH] x86/FPU: Fix FPU handling on legacy FPU machines

From: Borislav Petkov
Date: Fri Mar 11 2016 - 06:32:33 EST

486 cores like Intel Quark support only the very old, legacy x87 FPU
(FSAVE/FRSTOR, CPUID bit FXSR is not set). And our FPU code wasn't
handling the saving and restoring there properly. First, Andy Shevchenko
reported a splat:

WARNING: CPU: 0 PID: 823 at arch/x86/include/asm/fpu/internal.h:163 fpu__clear+0x8c/0x160

which was us trying to execute FXRSTOR on those machines even though
they don't support it.

After taking care of that, Bryan O'Donoghue reported that a simple FPU
test still failed because we weren't initializing the FPU state properly
on those machines.

Take care of all that.

Reported-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
Reported-and-tested-by: Bryan O'Donoghue <pure.logic@xxxxxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
arch/x86/kernel/fpu/core.c | 4 +++-
arch/x86/kernel/fpu/init.c | 2 +-
2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index dea8e76d60c6..8e37cc8a539a 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -474,8 +474,10 @@ static inline void copy_init_fpstate_to_fpregs(void)
if (use_xsave())
copy_kernel_to_xregs(&init_fpstate.xsave, -1);
- else
+ else if (static_cpu_has(X86_FEATURE_FXSR))
+ else
+ copy_kernel_to_fregs(&init_fpstate.fsave);

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index e12cc0ad368e..c835f61d5feb 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -134,7 +134,7 @@ static void __init fpu__init_system_generic(void)
* Set up the legacy init FPU context. (xstate init might overwrite this
* with a more modern format, if the CPU supports it.)
- fpstate_init_fxstate(&init_fpstate.fxsave);
+ fpstate_init(&init_fpstate);



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