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Date:   Wed, 7 Oct 2020 08:25:45 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc:     Jethro Beekman <jethro@fortanix.com>, x86@kernel.org,
        linux-sgx@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Cedric Xing <cedric.xing@intel.com>, akpm@linux-foundation.org,
        andriy.shevchenko@linux.intel.com, asapek@google.com, bp@alien8.de,
        chenalexchen@google.com, conradparker@google.com,
        cyhanish@google.com, dave.hansen@intel.com, haitao.huang@intel.com,
        kai.huang@intel.com, kai.svahn@intel.com, kmoy@google.com,
        ludloff@google.com, luto@kernel.org, nhorman@redhat.com,
        npmccallum@redhat.com, puiterwijk@redhat.com, rientjes@google.com,
        tglx@linutronix.de, yaozhangx@google.com, mikko.ylinen@intel.com
Subject: Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
Message-ID: <20201007152545.GB758@linux.intel.com>
References: <20201006025703.GG15803@linux.intel.com>
 <453c2d9b-0fd0-0d63-2bb9-096f255a6ff4@fortanix.com>
 <20201006151532.GA17610@linux.intel.com>
 <20201006172819.GA114208@linux.intel.com>
 <20201006232129.GB28981@linux.intel.com>
 <20201007002236.GA139112@linux.intel.com>
 <20201007011738.GE28981@linux.intel.com>
 <20201007031402.GA143690@linux.intel.com>
 <20201007043418.GG28981@linux.intel.com>
 <20201007073923.GA3632@linux.intel.com>
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On Wed, Oct 07, 2020 at 10:39:23AM +0300, Jarkko Sakkinen wrote:
> On Tue, Oct 06, 2020 at 09:34:19PM -0700, Sean Christopherson wrote:
> > > Even if that was in place, you'd need to separate normal and interrupt.
> > > Tristate is useless here. 
> > 
> > Huh?  You mean like adding SGX_INTERRUPT_EXIT and SGX_EXCEPTION_EXIT?
> 
> OK, so I'll throw something.
> 
> 1. "normal" is either exception from either EENTER or ERESUME,
>    or just EEXIT.
> 2. "interrupt" is something where you want to tailor AEP path.

Manipulating the behavior of the vDSO, as in #2, would be done via an input
flag.  It may be related to the exit reason, e.g. the flag may also opt-in to
a new exit reason, but that has no bearing on whether or not a dedicated exit
reason is valuable.

> > I'm not arguing that any of the above is even remotely likely.  I just don't
> > understand why we'd want an API that at best requires heuristics in userspace
> > to determine why the enclave stopped running, and at worst will saddle us with
> > an ugly mess in the future.  All to save 4 bytes that no one cares about (they
> > literally cost nothing), and a single MOV in a flow that is hundreds, if not
> > thousands, of cycles.
> 
> I don't care as much as saving bytes as defining API, which has zero
> ambiguous state variables.

How is exit_reason ambiguous?