Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Sean Christopherson]

On Mon, Mar 16, 2020 at 04:50:26PM -0700, Xing, Cedric wrote:
> On 3/16/2020 3:53 PM, Sean Christopherson wrote:
> >On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote:
> >>>My suggestions explicitly maintained robustness, and in fact increased
> >>>it. If you think we've lost capability, please speak with specificity
> >>>rather than in vague generalities. Under my suggestions we can:
> >>>1. call the vDSO from C
> >>>2. pass context to the handler
> >>>3. have additional stack manipulation options in the handler
> >>>
> >>>The cost for this is a net 2 additional instructions. No existing
> >>>capability is lost.
> >>
> >>My vague generality in this case is just that the whole design
> >>approach so far has been to minimize the amount of wrapping to
> >>EENTER.
> >
> >Yes and no.   If we wanted to minimize the amount of wrapping around the
> >vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the
> >first place.  The whole process has been about balancing the wants of each
> >use case against the overall quality of the API and code.
> >
> The design of this vDSO API was NOT to minimize wrapping, but to allow
> maximal flexibility. More specifically, we strove not to restrict how info
> was exchanged between the enclave and its host process. After all, calling
> convention is compiler specific - i.e. the enclave could be built by a
> different compiler (e.g. MSVC) that doesn't share the same list of CSRs as
> the host process. Therefore, the API has been implemented to pass through
> virtually all registers except those used by EENTER itself. Similarly, all
> registers are passed back from enclave to the caller (or the exit handler)
> except those used by EEXIT. %rbp is an exception because the vDSO API has to
> anchor the stack, using either %rsp or %rbp. We picked %rbp to allow the
> enclave to allocate space on the stack.

And unless I'm missing something, using %rcx to pass @leaf would still
satisfy the above, correct?  Ditto for saving/restoring %rbx.

I.e. a runtime that's designed to work with enclave's using a different
calling convention wouldn't be able to take advantage of being able to call
the vDSO from C, but neither would it take on any meaningful burden.