linux-kernel - Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Wed, 7 Oct 2020 08:25:45 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Jethro Beekman <jethro@...tanix.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Cedric Xing <cedric.xing@...el.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        chenalexchen@...gle.com, conradparker@...gle.com,
        cyhanish@...gle.com, dave.hansen@...el.com, haitao.huang@...el.com,
        kai.huang@...el.com, kai.svahn@...el.com, kmoy@...gle.com,
        ludloff@...gle.com, luto@...nel.org, nhorman@...hat.com,
        npmccallum@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com, mikko.ylinen@...el.com
Subject: Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

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?