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.
