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. /Jarkko
