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    Subject[PATCH 053/208] x86/fpu: Document fpu__unlazy_stopped()
    Date
    Explain its usage and also document a TODO item.

    Reviewed-by: Borislav Petkov <bp@alien8.de>
    Cc: Andy Lutomirski <luto@amacapital.net>
    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: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/kernel/fpu/core.c | 25 +++++++++++++++++++++----
    1 file changed, 21 insertions(+), 4 deletions(-)

    diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
    index b8e3dbbcdc16..0235df54cd48 100644
    --- a/arch/x86/kernel/fpu/core.c
    +++ b/arch/x86/kernel/fpu/core.c
    @@ -284,10 +284,27 @@ int fpstate_alloc_init(struct task_struct *curr)
    EXPORT_SYMBOL_GPL(fpstate_alloc_init);

    /*
    - * The _current_ task is using the FPU for the first time
    - * so initialize it and set the mxcsr to its default
    - * value at reset if we support XMM instructions and then
    - * remember the current task has used the FPU.
    + * This function is called before we modify a stopped child's
    + * FPU state context.
    + *
    + * If the child has not used the FPU before then initialize its
    + * FPU context.
    + *
    + * If the child has used the FPU before then unlazy it.
    + *
    + * [ After this function call, after the context is modified and
    + * the child task is woken up, the child task will restore
    + * the modified FPU state from the modified context. If we
    + * didn't clear its lazy status here then the lazy in-registers
    + * state pending on its former CPU could be restored, losing
    + * the modifications. ]
    + *
    + * This function is also called before we read a stopped child's
    + * FPU state - to make sure it's modified.
    + *
    + * TODO: A future optimization would be to skip the unlazying in
    + * the read-only case, it's not strictly necessary for
    + * read-only access to the context.
    */
    static int fpu__unlazy_stopped(struct task_struct *child)
    {
    --
    2.1.0


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