selftests/sgx: Introduce dynamic entry point

The test enclave (test_encl.elf) is built with two initialized
Thread Control Structures (TCS) included in the binary. Both TCS are
initialized with the same entry point, encl_entry, that correctly
computes the absolute address of the stack based on the stack of each
TCS that is also built into the binary.

A new TCS can be added dynamically to the enclave and requires to be
initialized with an entry point used to enter the enclave. Since the
existing entry point, encl_entry, assumes that the TCS and its stack
exists at particular offsets within the binary it is not able to handle
a dynamically added TCS and its stack.

Introduce a new entry point, encl_dyn_entry, that initializes the
absolute address of that thread's stack to the address immediately
preceding the TCS itself. It is now possible to dynamically add a
contiguous memory region to the enclave with the new stack preceding
the new TCS. With the new TCS initialized with encl_dyn_entry as entry
point the absolute address of the stack is computed correctly on entry.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/93e9c420dedf5f773ba6965c18245bc7d62aca83.1652137848.git.reinette.chatre@intel.com

tools/testing/selftests/sgx/test_encl_bootstrap.S

@@ -45,6 +45,12 @@ encl_entry:
 	# TCS #2. By adding the value of encl_stack to it, we get
 	# the absolute address for the stack.
 	lea	(encl_stack)(%rbx), %rax
+	jmp encl_entry_core
+encl_dyn_entry:
+	# Entry point for dynamically created TCS page expected to follow
+	# its stack directly.
+	lea -1(%rbx), %rax
+encl_entry_core:
 	xchg	%rsp, %rax
 	push	%rax