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Date:   Wed, 7 Oct 2020 20:08:18 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@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: <20201007170818.GB3885@linux.intel.com>
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 <20201006232129.GB28981@linux.intel.com>
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 <20201007011738.GE28981@linux.intel.com>
 <20201007031402.GA143690@linux.intel.com>
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Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
Precedence: bulk

On Wed, Oct 07, 2020 at 08:25:45AM -0700, Sean Christopherson wrote:
> 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.

The fact is that AEP path is not actual right now.

I'm not even interested to go further on discussing about feature that
does not exist. Perhaps if/when it exist it turns out that we want a
variable lets say 'exit_reason' to present something in that context.

I'm neither against that or for it because it is all speculative.

> > > 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?

I rather pick the word redundant:

1. 'leaf' exist anyway.
2. It can represent all the state we need right now.
3. It does not block anything.,

I care deeply about wasting 4 bytes in a fixed size struct for
absolutely nothing.

/Jarkko