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Date:   Thu, 11 Jul 2019 02:15:38 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     "Xing, Cedric" <cedric.xing@intel.com>
Cc:     linux-kernel@vger.kernel.org, linux-sgx@vger.kernel.org,
        akpm@linux-foundation.org, dave.hansen@intel.com,
        sean.j.christopherson@intel.com, serge.ayoun@intel.com,
        shay.katz-zamir@intel.com, haitao.huang@intel.com,
        kai.svahn@intel.com, kai.huang@intel.com
Subject: Re: [RFC PATCH v2 0/3] An alternative __vdso_sgx_enter_enclave() to
 allow enclave/host parameter passing using untrusted stack
Message-ID: <20190710231538.dkc7tyeyvns53737@linux.intel.com>
References: <cover.1555965327.git.cedric.xing@intel.com>
 <20190424062623.4345-1-cedric.xing@intel.com>
 <20190710111719.nnoedfo4wvbfghq7@linux.intel.com>
 <686e47d2-f45c-6828-39d1-48374925de6c@intel.com>
 <20190710224628.epjxwlpqqxdurmzo@linux.intel.com>
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On Thu, Jul 11, 2019 at 01:46:28AM +0300, Jarkko Sakkinen wrote:
> On Wed, Jul 10, 2019 at 11:08:37AM -0700, Xing, Cedric wrote:
> > > With these conclusions I think the current vDSO API is sufficient for
> > > Linux.
> > 
> > The new vDSO API is to support data exchange on stack. It has nothing to do
> > with debugging. BTW, the community has closed on this.
> 
> And how that is useful?
> 
> > The CFI directives are for stack unwinding. They don't affect what the code
> > does so you can just treat them as NOPs if you don't understand what they
> > do. However, they are useful to not only debuggers but also exception
> > handling code. libunwind also has a setjmp()/longjmp() implementation based
> > on CFI directives.
> 
> Of course I won't merge code of which usefulness I don't understand.

I re-read the cover letter [1] because it usually is the place
to "pitch" a feature.

It fails to address two things:

1. How and in what circumstances is an untrusted stack is a better
   vessel for handling exceptions than the register based approach
   that we already have?
2. How is it simpler approach? There is a strong claim of simplicity
   and convenience without anything backing it.
3. Why we need both register and stack based approach co-exist? I'd go
   with one approach for a new API without any legacy whatsoever.

This really needs a better pitch before we can consider doing anything
to it.

Also, in [2] there is talk about the next revision. Maybe the way go
forward is to address the three issues I found in the cover letter
and fix whatever needed to be fixed in the actual patches?

[1] https://lkml.org/lkml/2019/4/24/84
[2] https://lkml.org/lkml/2019/4/25/1170

/Jarkko