Add a configuration parameter to control the use of Metal SPIs.

With the new `MVK_CONFIG_USE_METAL_PRIVATE_API`, the user can prevent their
use at run time without recompiling MoltenVK. This may be useful for
troubleshooting purposes.

The extra member is because the compiler adds tail padding to make the
struct aligned on an 8-byte boundary, so the `static_assert()` fails
without it.

Docs/MoltenVK_Configuration_Parameters.md

@@ -663,3 +663,16 @@ Determines the style used to implement _Vulkan_ semaphore (`VkSemaphore`) functi
 In the special case of `VK_SEMAPHORE_TYPE_TIMELINE` semaphores, **MoltenVK** will always use
 `MTLSharedEvent` if it is available on the platform, regardless of the value of this parameter.
 
+
+---------------------------------------
+#### MVK_CONFIG_USE_METAL_PRIVATE_API
+
+##### Type: Boolean
+##### Default: Value of `MVK_USE_METAL_PRIVATE_API`
+
+If enabled, **MoltenVK** will _use_ private interfaces exposed by _Metal_ to implement _Vulkan_
+features that are difficult to support otherwise.
+
+Unlike `MVK_USE_METAL_PRIVATE_API`, this setting may be overridden at run time.
+
+This option is not available unless MoltenVK were built with `MVK_USE_METAL_PRIVATE_API` set to `1`.

MoltenVK/MoltenVK/API/mvk_private_api.h

@@ -240,6 +240,8 @@ typedef struct {
 	MVKConfigCompressionAlgorithm shaderSourceCompressionAlgorithm;            /**< MVK_CONFIG_SHADER_COMPRESSION_ALGORITHM */
 	VkBool32 shouldMaximizeConcurrentCompilation;                              /**< MVK_CONFIG_SHOULD_MAXIMIZE_CONCURRENT_COMPILATION */
 	float timestampPeriodLowPassAlpha;                                         /**< MVK_CONFIG_TIMESTAMP_PERIOD_LOWPASS_ALPHA */
+	VkBool32 useMetalPrivateAPI;                                               /**< MVK_CONFIG_USE_METAL_PRIVATE_API */
+	uint32_t _unused_struct_padding;
 } MVKConfiguration;
 
 // Legacy support for renamed struct elements.

MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm

@@ -2341,7 +2341,7 @@ void MVKPhysicalDevice::initFeatures() {
     _features.depthBiasClamp = true;
     _features.fillModeNonSolid = true;
     _features.largePoints = true;
-	_features.wideLines = static_cast<bool>(MVK_USE_METAL_PRIVATE_API);
+	_features.wideLines = getMVKConfig().useMetalPrivateAPI;
     _features.alphaToOne = true;
     _features.samplerAnisotropy = true;
     _features.shaderImageGatherExtended = true;

MoltenVK/MoltenVK/Utility/MVKConfigMembers.def

@@ -81,6 +81,8 @@ MVK_CONFIG_MEMBER(useMetalArgumentBuffers,                MVKUseMetalArgumentBuf
 MVK_CONFIG_MEMBER(shaderSourceCompressionAlgorithm,       MVKConfigCompressionAlgorithm,            SHADER_COMPRESSION_ALGORITHM)
 MVK_CONFIG_MEMBER(shouldMaximizeConcurrentCompilation,    VkBool32,                                 SHOULD_MAXIMIZE_CONCURRENT_COMPILATION)
 MVK_CONFIG_MEMBER(timestampPeriodLowPassAlpha,            float,                                    TIMESTAMP_PERIOD_LOWPASS_ALPHA)
+MVK_CONFIG_MEMBER(useMetalPrivateAPI,                     VkBool32,                                 USE_METAL_PRIVATE_API)
+MVK_CONFIG_MEMBER(_unused_struct_padding,                 uint32_t,                                 _UNUSED_STRUCT_PADDING)
 
 #undef MVK_CONFIG_MEMBER
 #undef MVK_CONFIG_MEMBER_STRING

MoltenVK/MoltenVK/Utility/MVKEnvironment.cpp

@@ -66,6 +66,9 @@ void mvkSetConfig(MVKConfiguration& dstMVKConfig, const MVKConfiguration& srcMVK
 	// Clamp timestampPeriodLowPassAlpha between 0.0 and 1.0.
 	dstMVKConfig.timestampPeriodLowPassAlpha = mvkClamp(dstMVKConfig.timestampPeriodLowPassAlpha, 0.0f, 1.0f);
 
+	// Only allow useMetalPrivateAPI to be enabled if we were built with support for it.
+	dstMVKConfig.useMetalPrivateAPI = dstMVKConfig.useMetalPrivateAPI && MVK_USE_METAL_PRIVATE_API;
+
 	// For each string member of the destination MVKConfiguration, store the contents
 	// in a std::string, then repoint the member to the contents of the std::string.
 #define MVK_CONFIG_MEMBER(member, mbrType, name)

MoltenVK/MoltenVK/Utility/MVKEnvironment.h

@@ -348,3 +348,14 @@ void mvkSetConfig(MVKConfiguration& dstMVKConfig, const MVKConfiguration& srcMVK
 #ifndef MVK_CONFIG_TIMESTAMP_PERIOD_LOWPASS_ALPHA
 #  	define MVK_CONFIG_TIMESTAMP_PERIOD_LOWPASS_ALPHA    1.0
 #endif
+
+/**
+ * Enable the use of Metal private interfaces, also known as "Service Provider Interfaces" (SPIs),
+ * to support Vulkan features. Enabled by default if support is included.
+ */
+#ifndef MVK_CONFIG_USE_METAL_PRIVATE_API
+#	define MVK_CONFIG_USE_METAL_PRIVATE_API MVK_USE_METAL_PRIVATE_API
+#endif
+
+#undef MVK_CONFIG__UNUSED_STRUCT_PADDING
+#define MVK_CONFIG__UNUSED_STRUCT_PADDING 0

README.md

@@ -317,9 +317,10 @@ _Vulkan_ API calls as function pointers.
 <a name="metal_private_api"></a>
 ### Accessing _Metal_ Private API calls
 
-You can optionally build **MoltenVK** with access to private _Metal_ API calls.
+You can optionally build **MoltenVK** with access to private _Metal_ API calls, also known
-Doing so will allow **MoltenVK** to extend its functionality by using certain private _Metal_ 
+as "Service Provider Interfaces" (SPIs). Doing so will allow **MoltenVK** to extend its
-API calls, but it will also disqualify the app from being distributed via _Apple_ App Stores.
+functionality by using certain private _Metal_ API calls, but it will also disqualify the
+app from being distributed via _Apple_ App Stores.
 
 To do so, when building **MoltenVK**, set the build setting `MVK_USE_METAL_PRIVATE_API=1`.
 This build setting can be set in the `MoltenVK.xcodeproj` *Xcode* project, or it can be