Build L0 v2 adapter by default (#17407)

L0 v2 adapter is expected to be shipped as part of 2025.2 release.

Refactor params to configure.py into a single 'choice' param.

Author: igchor

source/adapters/level_zero/v2/README.md

@@ -6,6 +6,14 @@ L0 v2 adapter can be enabled by setting passing `UR_BUILD_ADAPTER_L0_V2=1` optio
 
 Currently, L0 v2 is only available as an experimental adapter to gather feedback and adding missing features. L0 v2 is planned to be the default adapter for L0 in 2026.0 release.
 
+To enable L0 v2 adapter at runtime, set, `SYCL_UR_USE_LEVEL_ZERO_V2=1`.
+
+This forces UR and SYCL to use the v2 adapter instead of the legacy version.
+
+Alternatively, `UR_ADAPTER_FORCE_LOAD` env variable can be used.
+
+SYCL E2E tests can be used to test v2 adapter by passing `level_zero_v2:gpu` to llvm-lit `sycl_devices`.
+
 # Code structure
 
 v2 adapters is is a standalone adapter but reuses some logic from the legacy L0 adapter implementation - most notably: adapter.cpp, platform.cpp, device.cpp

source/loader/ur_adapter_registry.hpp

@@ -324,13 +324,15 @@ class AdapterRegistry {
       // Skip legacy L0 adapter if the v2 adapter is requested, and vice versa.
       if (std::string(adapterName).find("level_zero") != std::string::npos) {
         auto v2Requested = getenv_tobool("UR_LOADER_USE_LEVEL_ZERO_V2", false);
+        v2Requested |= getenv_tobool("SYCL_UR_USE_LEVEL_ZERO_V2", false);
         auto v2Adapter =
             std::string(adapterName).find("v2") != std::string::npos;
 
         if (v2Requested != v2Adapter) {
-          logger::info("The adapter '{}' is skipped because {} {}.",
-                       adapterName, "UR_LOADER_USE_LEVEL_ZERO_V2",
-                       v2Requested ? "is set" : "is not set");
+          logger::info(
+              "The adapter '{}' is skipped because {} {}.", adapterName,
+              "UR_LOADER_USE_LEVEL_ZERO_V2 or SYCL_UR_USE_LEVEL_ZERO_V2",
+              v2Requested ? "is set" : "is not set");
           continue;
         }
       }