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    SubjectRe: [PATCH] Make math_state_restore() save and restore the interrupt flag
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    On Mon, 2014-02-03 at 10:20 -0800, Linus Torvalds wrote:
    > Thinking about it some more, this patch is *almost* not needed at all.
    >
    > I'm wondering if you should just change the first patch to just always
    > initialize the fpu when it is allocated, and at execve() time (ie in
    > flush_thread()).
    >

    We already do this for eager-fpu case, in eager_fpu_init() during boot
    and in drop_init_fpu() during flush_thread().

    > If we do that, then this:
    >
    > + if (!tsk_used_math(tsk))
    > + init_fpu(tsk);
    >
    > can be dropped entirely from math_state_restore().

    yeah, probably for eager-fpu, but:

    > And quite frankly,
    > at that point, I think all the changes to __kernel_fpu_end() can go
    > away, because at that point math_state_restore() really does the right
    > thing - all the allocations are gone, and all the async task state
    > games are gone, only the "restore state" remains.
    >
    > Hmm? So the only thing needed would be to add that "init_fpu()" to the
    > initial bootmem allocation path and to change flush_thread() (it
    > currently does "drop_init_fpu()", let's just make it initialize the
    > FPU state using fpu_finit()), and then we could remove the whole
    > "used_math" bit entirely, and just say that the FPU is always
    > initialized.
    >
    > What do you guys think?

    No. as I mentioned in the changelog, there is one more path which does
    drop_fpu() and we still depend on this used_math bit for eager-fpu.

    in signal restore path for 32-bit app, where we copy the sig-context
    state from the user stack to the kernel manually (because of legacy
    reasons where fsave state is followed by fxsave state etc in the 32-bit
    signal handler context and we have to go through convert_to_fxsr() etc).

    from __restore_xstate_sig() :

    /*
    * Drop the current fpu which clears used_math(). This ensures
    * that any context-switch during the copy of the new state,
    * avoids the intermediate state from getting restored/saved.
    * Thus avoiding the new restored state from getting corrupted.
    * We will be ready to restore/save the state only after
    * set_used_math() is again set.
    */
    drop_fpu(tsk);


    thanks,
    suresh



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