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

[Jarkko Sakkinen]

On Mon, Mar 16, 2020 at 04:56:42PM -0700, Xing, Cedric wrote:
> On 3/16/2020 3:55 PM, Sean Christopherson wrote:
> > On Mon, Mar 16, 2020 at 02:31:36PM +0100, Jethro Beekman wrote:
> > > Can someone remind me why we're not passing TCS in RBX but on the stack?
> > 
> > I finally remembered why.  It's pulled off the stack and passed into the
> > exit handler.  I'm pretty sure the vDSO could take it in %rbx and manually
> > save it on the stack, but I'd rather keep the current behavior so that the
> > vDSO is callable from C (assuming @leaf is changed to be passed via %rcx).
> > 
> The idea is that the caller of this vDSO API is C callable, hence it cannot
> receive TCS in %rbx anyway. Then it has to either MOV to %rbx or PUSH to
> stack. Either way the complexity is the same. The vDSO API however has to
> always save it on stack for exit handler. So receiving it via stack ends up
> in simplest code.

It is never C callable anyway given that not following the ABI so
who cares about being C callable anyway?

/Jarkko