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    SubjectRe: Lazy FPU restoration / moving kernel_fpu_end() to context switch
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    On 06/15/2018 01:33 PM, Jason A. Donenfeld wrote:
    > On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@kernel.org> wrote:
    >> quite in the form you imagined. The idea that we've tossed around is
    >> to restore FPU state on return to user mode. Roughly, we'd introduce
    >> a new thread flag TIF_FPU_UNLOADED (name TBD).
    >> prepare_exit_to_usermode() would notice this flag, copy the fpstate to
    >> fpregs, and clear the flag. (Or maybe exit_to_usermode_loop() -- No
    >> one has quite thought it through, but I think it should be outside the
    >> loop.) We'd update all the FPU accessors to understand the flag.
    > Yes! This is exactly what I was thinking. Then those calls to begin()
    > and end() could be placed as close to the actual FPU usage as
    > possible.

    Andy, what was the specific concern about PKRU? That we might do:

    kernel_fpu_begin(); <- Saves the first time
    something()
    kernel_fpu_end(); <- Does not XRSTOR

    copy_from_user(); <- Sees old PKRU, does the wrong thing

    prepare_exit_to_usermode(); <- Does the XRSTOR
    // only now does PKRU have the right value
    SYSRET/IRET

    ?

    Does that *matter* unless something() modified PKRU? We could just make
    the rule that nobody is supposed to mess with it and that it's not
    covered by kernel_fpu_begin/end() semantics. We could even
    theoretically enforce that in a debug environment if we watch its value.

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