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Subject: Re: RFC: userspace exception fixups
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rich Felker <dalias@libc.org>, Jann Horn <jannh@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
        Linux API <linux-api@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, nhorman@redhat.com,
        npmccallum@redhat.com, "Ayoun, Serge" <serge.ayoun@intel.com>,
        shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Carlos O'Donell <carlos@redhat.com>,
        adhemerval.zanella@linaro.org
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <a4d2b3ec-43ec-f062-e180-c6e5a0d9fab8@intel.com>
Date:   Fri, 2 Nov 2018 10:13:23 -0700
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On 11/2/18 10:06 AM, Sean Christopherson wrote:
> On Fri, Nov 02, 2018 at 09:56:44AM -0700, Dave Hansen wrote:
>> On 11/2/18 9:30 AM, Sean Christopherson wrote:
>>> What if rather than having userspace register an address for fixup, the
>>> kernel instead unconditionally does fixup on the ENCLU opcode?
>>
>> The problem is knowing what to do for the fixup.  If we have a simple
>> action to take that's universal, like backing up %RIP, or setting some
>> other register state, it's not bad.
> 
> Isn't the EENTER/RESUME behavior universal?  Or am I missing something?

Could someone write down all the ways we get in and out of the enclave?

I think we always get in from userspace calling EENTER or ERESUME.  We
can't ever enter directly from the kernel, like via an IRET from what I
understand.

We get *out* from exceptions, hardware interrupts, or enclave-explicit
EEXITs.  Did I miss any?  Remind me where the hardware lands the control
flow in each of those exit cases.