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    SubjectRe: RFC: userspace exception fixups
    On Fri, Nov 02, 2018 at 04:56:36PM +0000, Jethro Beekman wrote:
    > On 2018-11-02 09:52, Sean Christopherson wrote:
    > >On Fri, Nov 02, 2018 at 04:37:10PM +0000, Jethro Beekman wrote:
    > >>On 2018-11-02 09:30, Sean Christopherson wrote:
    > >>>... The intended convention for EENTER is to have an ENCLU at the AEX target ...
    > >>>
    > >>>... to further enforce that the AEX target needs to be ENCLU.
    > >>
    > >>Some SGX runtimes may want to use a different AEX target.
    > >
    > >To what end? Userspace gets no indication as to why the AEX occurred.
    > >And if exceptions are getting transfered to userspace the trampoline
    > >would effectively be handling only INTR, NMI, #MC and EPC #PF.
    > >
    >
    > Various reasons...
    >
    > Userspace may have established an exception handling convention with the
    > enclave (by setting TCS.NSSA > 1) and may want to call EENTER instead of
    > ERESUME.

    The ERESUME trampoline would only be invoked for exceptions that aren't
    transferred to userspace. On #BR, #UD, etc..., the kernel would fixup
    %RIP to effectively point at @fault_handler. Userspace can then do
    whatever it wants to handle the fault, e.g. do EENTER if the fault needs
    to be serviced by the enclave.

    > Userspace may want fine-grained control over enclave scheduling (e.g.
    > SGX-Step)

    Uh, isn't SGX-Step an attack on SGX? Preventing userspace from playing
    games with enclave scheduling seems like a good thing.

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