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Subject[PATCH 4.4 12/24] x86/fpu: Fix math emulation in eager fpu mode
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@kernel.org>

commit 4ecd16ec7059390b430af34bd8bc3ca2b5dcef9a upstream.

Systems without an FPU are generally old and therefore use lazy FPU
switching. Unsurprisingly, math emulation in eager FPU mode is a
bit buggy. Fix it.

There were two bugs involving kernel code trying to use the FPU
registers in eager mode even if they didn't exist and one BUG_ON()
that was incorrect.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
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: Rik van Riel <riel@redhat.com>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: yu-cheng yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/b4b8d112436bd6fab866e1b4011131507e8d7fbe.1453675014.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/fpu/internal.h | 3 ++-
arch/x86/kernel/fpu/core.c | 2 +-
arch/x86/kernel/traps.c | 1 -
3 files changed, 3 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -596,7 +596,8 @@ switch_fpu_prepare(struct fpu *old_fpu,
* If the task has used the math, pre-load the FPU on xsave processors
* or if the past 5 consecutive context-switches used math.
*/
- fpu.preload = new_fpu->fpstate_active &&
+ fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
+ new_fpu->fpstate_active &&
(use_eager_fpu() || new_fpu->counter > 5);

if (old_fpu->fpregs_active) {
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -437,7 +437,7 @@ void fpu__clear(struct fpu *fpu)
{
WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */

- if (!use_eager_fpu()) {
+ if (!use_eager_fpu() || !static_cpu_has(X86_FEATURE_FPU)) {
/* FPU state will be reallocated lazily at the first use. */
fpu__drop(fpu);
} else {
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -751,7 +751,6 @@ dotraplinkage void
do_device_not_available(struct pt_regs *regs, long error_code)
{
RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
- BUG_ON(use_eager_fpu());

#ifdef CONFIG_MATH_EMULATION
if (read_cr0() & X86_CR0_EM) {

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