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

[Nathaniel McCallum]

On Mon, Mar 16, 2020 at 9:59 AM Jethro Beekman <jethro@xxxxxxxxxxxx> wrote:
>
> On 2020-03-16 14:57, Nathaniel McCallum wrote:
> > On Mon, Mar 16, 2020 at 9:32 AM Jethro Beekman <jethro@xxxxxxxxxxxx> wrote:
> >>
> >> On 2020-03-15 18:53, Nathaniel McCallum wrote:
> >>> On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen
> >>> <jarkko.sakkinen@xxxxxxxxxxxxxxx> 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.
> >>
> >> I'm not sure they're sensible? By departing from the ENCLU calling convention, both the VDSO
> >> and the wrapper become more complicated.
> >
> > For the vDSO, only marginally. I'm counting +4,-2 instructions in my
> > suggestions. For the wrapper, things become significantly simpler.
> >
> >> The wrapper because now it needs to implement all
> >> kinds of logic for different behavior depending on whether the VDSO is or isn't available.
> >
> > When isn't the vDSO available?
>
> When you're not on Linux. Or when you're on an old kernel.

I fail to see why the Linux kernel should degrade its new interfaces
for those use cases.