Re: RFC: userspace exception fixups

[Sean Christopherson]

On Thu, Nov 08, 2018 at 01:50:31PM -0800, Dave Hansen wrote:
> On 11/8/18 1:16 PM, Sean Christopherson wrote:
> > On Thu, Nov 08, 2018 at 12:10:30PM -0800, Dave Hansen wrote:
> >> On 11/8/18 12:05 PM, Andy Lutomirski wrote:
> >>> Hmm.  The idea being that the SDK preserves RBP but not RSP.  That's
> >>> not the most terrible thing in the world.  But could the SDK live with
> >>> something more like my suggestion where the vDSO supplies a normal
> >>> function that takes a struct containing registers that are visible to
> >>> the enclave?  This would make it extremely awkward for the enclave to
> >>> use the untrusted stack per se, but it would make it quite easy (I
> >>> think) for the untrusted part of the SDK to allocate some extra memory
> >>> and just tell the enclave that *that* memory is the stack.
> >>
> >> I really think the enclave should keep its grubby mitts off the
> >> untrusted stack.  There are lots of ways to get memory, even with
> >> stack-like semantics, that don't involve mucking with the stack itself.
> >>
> >> I have not heard a good, hard argument for why there is an absolute
> >> *need* to store things on the actual untrusted stack.
> > 
> > Convenience and performance are the only arguments I've heard, e.g. so
> > that allocating memory doesn't require an extra EEXIT->EENTER round trip.
> 
> Well, for the first access, it's going to cost a bunch asynchronous
> exits to fault in all the stack pages.  Instead of that, if you had a
> single area, or an explicit out-call to allocate and populate the area,
> you could do it in a single EEXIT and zero asynchronous exits for demand
> page faults.
> 
> So, it might be convenient, but I'm rather suspicious of any performance
> arguments.

Ya, I meant versus doing an EEXIT on every allocation, i.e. a very
naive allocation scheme.