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

[Nathaniel McCallum <npmccallum@redhat.com>]

On Mon, Mar 16, 2020 at 6:53 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote:
> > On Mon, 2020-03-16 at 10:01 -0400, Nathaniel McCallum wrote:
> > > On Mon, Mar 16, 2020 at 9:56 AM Jarkko Sakkinen
> > > <jarkko.sakkinen@linux.intel.com> wrote:
> > > > On Sun, 2020-03-15 at 13:53 -0400, Nathaniel McCallum wrote:
> > > > > On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen
> > > > > <jarkko.sakkinen@linux.intel.com> wrote:
> > > > > > On Wed, Mar 11, 2020 at 01:30:07PM -0400, Nathaniel McCallum wrote:
> > > > > > > Currently, the selftest has a wrapper around
> > > > > > > __vdso_sgx_enter_enclave() which preserves all x86-64 ABI callee-saved
> > > > > > > registers (CSRs), though it uses none of them. Then it calls this
> > > > > > > function which uses %rbx but preserves none of the CSRs. Then it jumps
> > > > > > > into an enclave which zeroes all these registers before returning.
> > > > > > > Thus:
> > > > > > >
> > > > > > >   1. wrapper saves all CSRs
> > > > > > >   2. wrapper repositions stack arguments
> > > > > > >   3. __vdso_sgx_enter_enclave() modifies, but does not save %rbx
> > > > > > >   4. selftest zeros all CSRs
> > > > > > >   5. wrapper loads all CSRs
> > > > > > >
> > > > > > > I'd like to propose instead that the enclave be responsible for saving
> > > > > > > and restoring CSRs. So instead of the above we have:
> > > > > > >   1. __vdso_sgx_enter_enclave() saves %rbx
> > > > > > >   2. enclave saves CSRs
> > > > > > >   3. enclave loads CSRs
> > > > > > >   4. __vdso_sgx_enter_enclave() loads %rbx
> > > > > > >
> > > > > > > I know that lots of other stuff happens during enclave transitions,
> > > > > > > but at the very least we could reduce the number of instructions
> > > > > > > through this critical path.
> > > > > >
> > > > > > What Jethro said and also that it is a good general principle to cut
> > > > > > down the semantics of any vdso as minimal as possible.
> > > > > >
> > > > > > I.e. even if saving RBX would make somehow sense it *can* be left
> > > > > > out without loss in terms of what can be done with the vDSO.
> > > > >
> > > > > Please read the rest of the thread. Sean and I have hammered out some
> > > > > sensible and effective changes.
> > > >
> > > > Have skimmed through that discussion but it comes down how much you get
> > > > by obviously degrading some of the robustness. Complexity of the calling
> > > > pattern is not something that should be emphasized as that is something
> > > > that is anyway hidden inside the runtime.
> > >
> > > 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.
>
> > And since this has been kind of agreed by most of the
> > stakeholders doing something against the chosen strategy is
> > something I do hold some resistance.
>
> Up until Nathaniel joined the party, the only stakeholder in terms of the
> exit handler was the Intel SDK.

I would hope that having additional stakeholders would ease the path
to adoption.

> There was a general consensus to pass
> registers as-is when there isn't a strong reason to do otherwise.  Note
> that Nathaniel has also expressed approval of that approach.

I still approve that approach.

> So I think the question that needs to be answered is whether the benefits
> of using %rcx instead of %rax to pass @leaf justify the "pass registers
> as-is" guideline.  We've effectively already given this waiver for %rbx,
> as the whole reason why the TCS is passed in on the stack instead of via
> %rbx is so that it can be passed to the exit handler.  E.g. the vDSO
> could take the TCS in %rbx and save it on the stack, but we're throwing
> the baby out with the bathwater at that point.
>
> The major benefits being that the vDSO would be callable from C and that
> the kernel could define a legitimate prototype instead of a frankenstein
> prototype that's half assembly and half C.  For me, those are significant
> benefits and well worth the extra MOV, PUSH and POP.  For some use cases
> it would eliminate the need for an assembly wrapper.  For runtimes that
> need an assembly wrapper for whatever reason, it's probably still a win as
> a well designed runtime can avoid register shuffling in the wrapper.  And
> if there is a runtime that isn't covered by the above, it's at worst an
> extra MOV.
>