Re: [RFC PATCH v4 5/5] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

[Sean Christopherson]

On Fri, Dec 14, 2018 at 07:12:04AM -0800, Sean Christopherson wrote:
> On Fri, Dec 14, 2018 at 09:55:49AM +0000, Jethro Beekman wrote:
> > On 2018-12-14 03:01, Sean Christopherson wrote:
> > >+2:	pop	%rbx
> > >+	pop	%r12
> > >+	pop	%r13
> > >+	pop	%r14
> > >+	pop	%r15
> > >+	pop	%rbp
> > >+	ret
> > 
> > x86-64 ABI requires that you call CLD here (enclave may set it).
> 
> Ugh.  Technically MXCSR and the x87 CW also need to be preserved.
> 
> What if rather than treating the enclave as hostile we require it to be
> compliant with the x86-64 ABI like any other function?  That would solve
> the EFLAGS.DF, MXCSR and x87 issues without adding unnecessary overhead.
> And we wouldn't have to save/restore R12-R15.  It'd mean we couldn't use
> the stack's red zone to hold @regs and @e, but that's poor form anyways.

Grr, except the processor crushes R12-R15, FCW and MXCSR on asynchronous
exits.  But not EFLAGS.DF, that's real helpful.