On Fri, Dec 14, 2018 at 09:03:11AM -0800, Sean Christopherson wrote: > On Fri, Dec 14, 2018 at 07:38:30AM -0800, Sean Christopherson wrote: > > 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. > > I can think of three options that are at least somewhat reasonable: > > 1) Save/restore MXCSR and FCW > > + 100% compliant with the x86-64 ABI > + Callable from any code > + Minimal documentation required > - Restoring MXCSR/FCW is likely unnecessary 99% of the time > - Slow > > 2) Clear EFLAGS.DF but not save/restore MXCSR and FCW > > + Mostly compliant with the x86-64 ABI > + Callable from any code that doesn't use SIMD registers > - Need to document deviations from x86-64 ABI > > 3) Require the caller to save/restore everything. > > + Fast > + Userspace can pass all GPRs to the enclave (minus EAX, RBX and RCX) > - Completely custom ABI > - For all intents and purposes must be called from an assembly wrapper > > > Option (3) actually isn't all that awful. RCX can be used to pass an > optional pointer to a 'struct sgx_enclave_exception' and we can still > return standard error codes, e.g. -EFAULT. Entering and exiting a syscall requires an assembly wrapper, and that doesn't seem completely unreasonable. It's an easy bit of inline assembly.
