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    Subject[tip:x86/urgent] x86/fpu: Fix i486 + no387 boot crash by only saving FPU registers on context switch if there is an FPU
    Commit-ID:  2224d616528194b02424c91c2ee254b3d29942c3
    Gitweb: https://git.kernel.org/tip/2224d616528194b02424c91c2ee254b3d29942c3
    Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    AuthorDate: Tue, 16 Oct 2018 22:25:25 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Wed, 17 Oct 2018 12:30:38 +0200

    x86/fpu: Fix i486 + no387 boot crash by only saving FPU registers on context switch if there is an FPU

    Booting an i486 with "no387 nofxsr" ends with with the following crash:

    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
    as the FPU is turned off.

    This bug was introduced in:

    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 in switch_fpu_finish() already).

    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Reviewed-by: Andy Lutomirski <luto@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: stable@vger.kernel.org
    Fixes: f1c8cd0176078 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active")
    Link: http://lkml.kernel.org/r/20181016202525.29437-4-bigeasy@linutronix.de
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    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 a38bf5a1e37a..69dcdf195b61 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
    \
     
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