Re: [RFC PATCH 4/4] x86/vdso: Add __vdso_sgx_eenter() to wrap SGX enclave transitions

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

On Wed, Dec 05, 2018 at 03:40:48PM -0800, Andy Lutomirski wrote:
> On Wed, Dec 5, 2018 at 3:20 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> > +notrace long __vdso_sgx_eenter(void *tcs, void *priv,
> > +                              struct sgx_eenter_fault_info *fault_info)
> > +{
> > +       u32 trapnr, error_code;
> > +       long leaf;
> > +       u64 addr;
> > +
> > +       /*
> > +        *      %eax = EENTER
> > +        *      %rbx = tcs
> > +        *      %rcx = do_eresume
> > +        *      %rdi = priv
> > +        * do_eenter:
> > +        *      enclu
> > +        *      jmp     out
> > +        *
> > +        * do_eresume:
> > +        *      enclu
> > +        *      ud2
> 
> Is the only reason for do_eresume to be different from do_eenter so
> that you can do the ud2?

No, it was a holdover from doing fixup via a magic prefix in user code.
The fixup could only skip the ENCLU and so a second ENCLU was needed to
differentiate between EENTER and ERESUME.  The need for two ENCLUs got
ingrained in my head.  I can't think of anything that will break if we
use a single ENCLU.

> > +        *
> > +        * out:
> > +        *      <return to C code>
> > +        *
> > +        * fault_fixup:
> > +        *      <extable loads RDI, DSI and RDX with fault info>
> > +        *      jmp     out
> > +        */
> 
> This has the IMO excellent property that it's extremely awkward to use
> it for a model where the enclave is reentrant.  I think it's excellent
> because reentrancy on the same enclave thread is just asking for
> severe bugs.  Of course, I fully expect the SDK to emulate reentrancy,
> but then it's 100% their problem :)  On the fiip side, it means that
> you can't really recover from a reported fault, even if you want to,
> because there's no way to ask for ERESUME.  So maybe the API should
> allow that after all.

Doh.  The ability to do ERESUME is an explicit requirement from the SDK
folks.  More code that I pulled from my userspace implementation and
didn't revisit.

> I think it might be polite to at least give some out regs, maybe RSI and RDI?

For the outbound path?  I was thinking @priv would be used for passing
data out as well as in.