> On Dec 14, 2018, at 9:03 AM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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. I like 3, but: > > E.g.: > > /** > * __vdso_sgx_enter_enclave() - Enter an SGX enclave > * > * %eax: ENCLU leaf, must be EENTER or ERESUME > * %rbx: TCS, must be non-NULL > * %rcx: Optional pointer to 'struct sgx_enclave_exception' > * > * Return: > * 0 on a clean entry/exit to/from the enclave > * -EINVAL if ENCLU leaf is not allowed or if TCS is NULL > * -EFAULT if ENCLU or the enclave faults > */ > ENTRY(__vdso_sgx_enter_enclave) > /* EENTER <= leaf <= ERESUME */ > cmp $0x2, %eax > jb bad_input > > cmp $0x3, %eax > ja bad_input > > /* TCS must be non-NULL */ > test %rbx, %rbx > je bad_input > > /* save @exception pointer */ > push %rcx > > /* load leaf, TCS and AEP for ENCLU */ > lea 1f(%rip), %rcx > 1: enclu > > add 0x8, %rsp > xor %eax, %eax > ret > > bad_input: > mov $(-EINVAL), %rax > ret > > .pushsection .fixup, "ax" > 2: pop %rcx > test %rcx, %rcx > je 3f > > mov %eax, EX_LEAF(%rcx) > mov %di, EX_TRAPNR(%rcx) > mov %si, EX_ERROR_CODE(%rcx) > mov %rdx, EX_ADDRESS(%rcx) > 3: mov $(-EFAULT), %rax > ret I’m not totally sold on -EFAULT as the error code. That usually indicates a bad pointer. I’m not sure I have a better suggestion. > .popsection > > _ASM_VDSO_EXTABLE_HANDLE(1b, 3b) > > ENDPROC(__vdso_sgx_enter_enclave)
