linux-kernel - Re: RFC: userspace exception fixups

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Date:   Fri, 2 Nov 2018 10:01:08 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jethro Beekman <jethro@...tanix.com>
Cc:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>, Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Carlos O'Donell" <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On Fri, Nov 2, 2018 at 9:56 AM Jethro Beekman <jethro@...tanix.com> 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.
>

Ugh,

I sincerely hope that a future ISA extension lets the kernel return
directly back to enclave mode so that AEX events become entirely
invisible to user code.  It would be nice if user developers didn't
start depending on the rather odd AEX semantics right now.  But I
don't think the kernel can sanely do much about it.