Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Dec 11, 2018 at 08:53:39AM -0600, Dr. Greg wrote:
> On Mon, Dec 10, 2018 at 03:24:50PM -0800, Josh Triplett wrote:
> 
> Good morning to everyone, I hope the week is progressing well.
> 
> > On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
> > > At that point I realized it's a hell of a lot easier to simply provide
> > > an IOCTL via /dev/sgx that allows userspace to register a per-process
> > > ENCLU exception handler.  At a high level, the basic idea is the same
> > > as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
> > > attempt to fixup select unhandled exceptions that occurred in user code.
> 
> > So, on the one hand, this is *absolutely* much cleaner than the VDSO
> > approach. On the other hand, this is global process state and has
> > some of the same problems as a signal handler as a result.
> 
> Sean's architecture is very simple and straight forward and thus has a
> lot going for it.
> 
> As Sean's approach indicates, by linking the exception handler to
> current->mm, SGX is very much a per memory map concept.  The issue is
> that there can be multiple enclaves loaded and excecuting in a
> processes memory map, the problem is, execution and thus exception
> handling, is very much at the per thread level.

Right, but is there a need to have a per-thread code page?  The handler
isn't per-process any more than the AEP is per-process.