[PATCH for_v29 3/8] x86/sgx: vdso: Make __vdso_sgx_enter_enclave() callable from C code

[Sean Christopherson <sean.j.christopherson@intel.com>]

Make __vdso_sgx_enter_enclave() callable from C by preserving %rbx
and taking @leaf in %rcx instead of %rax.  Being able to invoke the vDSO
from C reduces the overhead of runtimes that are tightly coupled with
their enclaves, e.g. that can rely on the enclave to save and restore
non-volatile registers, as the runtime doesn't need an assembly wrapper
to preserve non-volatile registers and/or shuffle stack arguments.

Note, both %rcx and %rbx are consumed by EENTER/ERESUME, i.e. consuming
them doesn't violate the primary tenet of __vdso_sgx_enter_enclave()
that "thou shalt not restrict how information is exchanged between an
enclave and its host process".

Suggested-by: Nathaniel McCallum <npmccallum@redhat.com>
Cc: Cedric Xing <cedric.xing@intel.com>
Cc: Jethro Beekman <jethro@fortanix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: linux-sgx@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---

Cedric, please go to town on this, you're much better at the userspace
stack and debugging interactions, e.g. I don't think the CFA directives
need updating, but my knowledge of that stuff is skeeetchy.

Note, if the previous patch to make the %rsp restoration relative is
dropped, then %rbx should be restored relative to %rbp instead of %rsp.

 arch/x86/entry/vdso/vsgx_enter_enclave.S | 33 ++++++++++++++----------
 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
index 14f07d5e47ae..7a0565476a29 100644
--- a/arch/x86/entry/vdso/vsgx_enter_enclave.S
+++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
@@ -16,25 +16,25 @@
 
 /**
  * __vdso_sgx_enter_enclave() - Enter an SGX enclave
+ * @rdi:	Pass-through value for RDI
+ * @rsi:	Pass-through value for RSI
+ * @rdx:	Pass-through value for RDX
  * @leaf:	ENCLU leaf, must be EENTER or ERESUME
+ * @r8:		Pass-through value for R8
+ * @r9:		Pass-through value for R9
  * @tcs:	TCS, must be non-NULL
  * @e:		Optional struct sgx_enclave_exception instance
  * @handler:	Optional enclave exit handler
  *
- * **Important!**  __vdso_sgx_enter_enclave() is **NOT** compliant with the
- * x86-64 ABI, i.e. cannot be called from standard C code.
- *
- * Input ABI:
- *  @leaf	%eax
- *  @tcs	8(%rsp)
- *  @e 		0x10(%rsp)
- *  @handler	0x18(%rsp)
- *
- * Output ABI:
- *  @ret	%eax
+ * **Important!**  __vdso_sgx_enter_enclave() does not ensure full compliance
+ * with the x86-64 ABI, e.g. doesn't explicitly clear EFLAGS.DF after EEXIT.
+ * Except for non-volatile general purpose registers, preserving/setting state
+ * in accordance with the x86-64 ABI is the responsibility of the enclave and
+ * its runtime, i.e. __vdso_sgx_enter_enclave() cannot be called from C code
+ * without careful consideration by both the enclave and its runtime.
  *
  * All general purpose registers except RAX, RBX and RCX are passed as-is to
- * the enclave. RAX, RBX and RCX are consumed by EENTER and ERESUME and are
+ * the enclave.  RAX, RBX and RCX are consumed by EENTER and ERESUME and are
  * loaded with @leaf, asynchronous exit pointer, and @tcs respectively.
  *
  * RBP and the stack are used to anchor __vdso_sgx_enter_enclave() to the
@@ -70,9 +70,11 @@
  */
 #ifdef SGX_KERNEL_DOC
 /* C-style function prototype to coerce kernel-doc into parsing the comment. */
-int __vdso_sgx_enter_enclave(int leaf, void *tcs,
+int __vdso_sgx_enter_enclave(unsigned long rdi, unsigned long rsi,
+			     unsigned long rdx, unsigned int leaf,
+			     unsigned long r8,  unsigned long r9, void *tcs,
 			     struct sgx_enclave_exception *e,
-			     sgx_enclave_exit_handler_t handler);
+			     sgx_enclave_exit_handler_t handler)
 #endif
 SYM_FUNC_START(__vdso_sgx_enter_enclave)
 	/* Prolog */
@@ -82,7 +84,9 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
 	.cfi_rel_offset		%rbp, 0
 	mov	%rsp, %rbp
 	.cfi_def_cfa_register	%rbp
+	push	%rbx
 
+	mov	%ecx, %eax
 .Lenter_enclave:
 	/* EENTER <= leaf <= ERESUME */
 	cmp	$0x2, %eax
@@ -108,6 +112,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave)
 	jne	.Linvoke_userspace_handler
 
 .Lout:
+	pop	%rbx
 	leave
 	.cfi_def_cfa		%rsp, 8
 	ret
-- 
2.24.1