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

[Sean Christopherson]

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?