Re: [RFC PATCH v2 4/4] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions

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

On Sat, Dec 08, 2018 at 08:15:38AM +0000, Jethro Beekman wrote:
> On 2018-12-08 00:14, Dave Hansen wrote:
> >On 12/7/18 10:15 AM, Jethro Beekman wrote:
> >>This is not sufficient to support the Fortanix SGX ABI calling
> >>convention, which was designed to be mostly compatible with the SysV
> >>64-bit calling convention. The following registers need to be passed in
> >>to an enclave from userspace: RDI, RSI, RDX, R8, R9, R10. The following
> >>registers need to be passed out from an enclave to userspace: RDI, RSI,
> >>RDX, R8, R9.
> >
> >Are you asking nicely to change the new Linux ABI to be consistent with
> >your existing ABI?  Or, are you saying that the new ABI *must* be
> >compatible with this previous out-of-tree implementation?
> 
> What's being discussed here is one of the alternatives for SGX fault
> handling, meant to improve the current status quo of having to use a signal
> handler.
> 
> I'm merely providing a data point that the currently proposed solution is
> not sufficient to support current use of the (ring 3) ENCLU instruction. You
> might find this useful in determining whether proposed kernel features will
> actually be used by users, and in further developing this solution or other
> solutions to the fault handling issue.
> 
> If going with the vDSO solution, I think something with semantics closer to
> the actual instruction would be preferred, like the following:
> 
> notrace __attribute__((naked)) long __vdso_sgx_enclu_with_aep()
> {
> 	asm volatile(
> 		"	lea	2f(%%rip), %%rcx\n"
> 		"1:	enclu\n"
> 		"2:     ret\n"
> 		".pushsection .fixup, \"ax\" \n"
> 		"3:	jmp 2b\n"
> 		".popsection\n"
> 		_ASM_VDSO_EXTABLE_HANDLE(1b, 3b)
> 		:::
> 	);
> }

Part of me likes this idea, but it's a documentation nightmare since
it's a completely customer register ABI.  And the caller's exception
handling gets a bit weird since RAX implicitly defines whether or not
an exception occurred.  I also think there's value in making the vDSO
function callable from standard C.