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

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