Async usercall interface for SGX enclaves

Credits for this commit go to:
Mohsen: fortanix#404
YxC: fortanix#441

This commit is an attempt to have the async-usercalls finally merged
into the main codebase (master branch).

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "fortanix-vme/tests/iron",
     "fortanix-vme/vme-pkix",
     "intel-sgx/aesm-client",
+    "intel-sgx/async-usercalls",
     "intel-sgx/dcap-provider",
     "intel-sgx/dcap-ql-sys",
     "intel-sgx/dcap-ql",

doc/generate-api-docs.sh

@@ -58,6 +58,9 @@ for LIB in $LIBS_SORTED; do
         if FEATURES="$(cargo read-manifest|jq -r '.metadata.docs.rs.features | join(",")' 2> /dev/null)"; then
             ARGS="--features $FEATURES"
         fi
+        if grep -q 'feature(sgx_platform)' ./src/lib.rs; then
+            ARGS+=" --target x86_64-fortanix-unknown-sgx"
+        fi
         cargo doc --no-deps --lib $ARGS
         popd
     fi

intel-sgx/async-usercalls/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "async-usercalls"
+version = "0.5.0"
+authors = ["Fortanix, Inc."]
+license = "MPL-2.0"
+edition = "2018"
+description = """
+An interface for asynchronous usercalls in SGX enclaves.
+This is an SGX-only crate, you should compile it with the `x86_64-fortanix-unknown-sgx` target.
+"""
+repository = "https://github.com/fortanix/rust-sgx"
+documentation = "https://edp.fortanix.com/docs/api/async_usercalls/"
+homepage = "https://edp.fortanix.com/"
+keywords = ["sgx", "async", "usercall"]
+categories = ["asynchronous"]
+
+[dependencies]
+# Project dependencies
+ipc-queue = { version = "0.2", path = "../../ipc-queue" }
+fortanix-sgx-abi = { version = "0.5.0", path = "../fortanix-sgx-abi" }
+
+# External dependencies
+lazy_static = "1.4.0"     # MIT/Apache-2.0
+crossbeam-channel = "0.5" # MIT/Apache-2.0
+fnv = "1.0"               # MIT/Apache-2.0
+
+# For cargo test --target x86_64-fortanix-unknown-sgx
+[package.metadata.fortanix-sgx]
+threads = 128