[PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode

[Andy Lutomirski]

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 <l...@kernel.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(-)

diff --git a/arch/x86/include/asm/fpu/internal.h 
b/arch/x86/include/asm/fpu/internal.h
index 0fd440df63f1..a1f78a9fbf41 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -589,7 +589,8 @@ switch_fpu_prepare(struct fpu *old_fpu, struct fpu 
*new_fpu, int cpu)
         * 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) {
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index d25097c3fc1d..08e1e11a05ca 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -423,7 +423,7 @@ 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 {
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index ade185a46b1d..87f80febf477 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -750,7 +750,6 @@ 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) {
-- 
2.5.0