VkHostImageCopyDevicePerformanceQueryEXT

To query if using VK_IMAGE_USAGE_HOST_TRANSFER_BIT_EXT has a negative impact on device performance when accessing an image, add VK_IMAGE_USAGE_HOST_TRANSFER_BIT_EXT to VkPhysicalDeviceImageFormatInfo2::usage, and add a VkHostImageCopyDevicePerformanceQueryEXT structure to the pNext chain of a VkImageFormatProperties2 structure passed to vkGetPhysicalDeviceImageFormatProperties2. This structure is defined as:

typedef struct VkHostImageCopyDevicePerformanceQueryEXT {
    VkStructureType sType;
    void* pNext;
    VkBool32 optimalDeviceAccess;
    VkBool32 identicalMemoryLayout;
} VkHostImageCopyDevicePerformanceQueryEXT;

sType is a VkStructureType value identifying this structure.
pNext is NULL or a pointer to a structure extending this structure.
optimalDeviceAccess returns VK_TRUE if use of host image copy has no adverse effect on device access performance, compared to an image that is created with exact same creation parameters, and bound to the same VkDeviceMemory, except that VK_IMAGE_USAGE_HOST_TRANSFER_BIT_EXT is replaced with VK_IMAGE_USAGE_TRANSFER_SRC_BIT and VK_IMAGE_USAGE_TRANSFER_DST_BIT.
identicalMemoryLayout returns VK_TRUE if use of host image copy has no impact on memory layout compared to an image that is created with exact same creation parameters, and bound to the same VkDeviceMemory, except that VK_IMAGE_USAGE_HOST_TRANSFER_BIT_EXT is replaced with VK_IMAGE_USAGE_TRANSFER_SRC_BIT and VK_IMAGE_USAGE_TRANSFER_DST_BIT.

The implementation may return VK_FALSE in optimalDeviceAccess if identicalMemoryLayout is VK_FALSE. If identicalMemoryLayout is VK_TRUE, optimalDeviceAccessmust be VK_TRUE.

The implementation may return VK_TRUE in optimalDeviceAccess while identicalMemoryLayout is VK_FALSE. In this situation, any device performance impact should not be measurable.

If VkPhysicalDeviceImageFormatInfo2::format is a block-compressed format and vkGetPhysicalDeviceImageFormatProperties2 returns VK_SUCCESS, the implementation must return VK_TRUE in optimalDeviceAccess.

Applications can make use of optimalDeviceAccess to determine their resource copying strategy. If a resource is expected to be accessed more on device than on the host, and the implementation considers the resource sub-optimally accessed, it is likely better to use device copies instead.

Layout not being identical yet still considered optimal for device access could happen if the implementation has different memory layout patterns, some of which are easier to access on the host.

The most practical reason for optimalDeviceAccess to be VK_FALSE is that host image access may disable framebuffer compression where it would otherwise have been enabled. This represents far more efficient host image access since no compression algorithm is required to read or write to the image, but it would impact device access performance. Some implementations may only set optimalDeviceAccess to VK_FALSE if certain conditions are met, such as specific image usage flags or creation flags.

Valid Usage (Implicit)

VUID-VkHostImageCopyDevicePerformanceQueryEXT-sType-sType

sType must be VK_STRUCTURE_TYPE_HOST_IMAGE_COPY_DEVICE_PERFORMANCE_QUERY_EXT