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    Subject[tip:x86/fpu] x86/fpu: Fix math emulation in eager fpu mode
    Commit-ID:  4ecd16ec7059390b430af34bd8bc3ca2b5dcef9a
    Gitweb: http://git.kernel.org/tip/4ecd16ec7059390b430af34bd8bc3ca2b5dcef9a
    Author: Andy Lutomirski <luto@kernel.org>
    AuthorDate: Sun, 24 Jan 2016 14:38:06 -0800
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Tue, 9 Feb 2016 15:42:55 +0100

    x86/fpu: Fix math emulation in eager fpu mode

    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>
    ---
    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 0fd440d..a1f78a9 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 d25097c..08e1e11 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 ade185a..87f80fe 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) {
    \
     
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