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

[Jarkko Sakkinen]

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