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Date:   Fri, 2 Nov 2018 10:12:51 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Jethro Beekman <jethro@fortanix.com>
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Subject: Re: RFC: userspace exception fixups
Message-ID: <20181102171251.GE7393@linux.intel.com>
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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.