linux-kernel - [PATCH 3/3] x86/fpu: Save FPU registers on context switch if there is a FPU

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Message-Id: <20181016202525.29437-4-bigeasy@linutronix.de>
Date:   Tue, 16 Oct 2018 22:25:25 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        stable@...r.kernel.org
Subject: [PATCH 3/3] x86/fpu: Save FPU registers on context switch if there is a FPU

Booting a 486 with "no387 nofxsr" ends with
| math_emulate: 0060:c101987d
| Kernel panic - not syncing: Math emulation needed in kernel

on the first context switch in user land. The reason is that
copy_fpregs_to_fpstate() tries `fnsave' which does not work. This
happens since commit f1c8cd0176078 ("x86/fpu: Change fpu->fpregs_active
users to fpu->fpstate_active").

Add a check for X86_FEATURE_FPU before trying to save FPU registers (we
have such a check switch_fpu_finish() already).

Fixes: f1c8cd0176078 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active")
Cc: stable@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/include/asm/fpu/internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index a38bf5a1e37ad..69dcdf195b611 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -528,7 +528,7 @@ static inline void fpregs_activate(struct fpu *fpu)
 static inline void
 switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
-	if (old_fpu->initialized) {
+	if (static_cpu_has(X86_FEATURE_FPU) && old_fpu->initialized) {
 		if (!copy_fpregs_to_fpstate(old_fpu))
 			old_fpu->last_cpu = -1;
 		else
-- 
2.19.1