Introduce KHR_external_semaphore_fd only for FD==-1#2670
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billhollings merged 1 commit intoKhronosGroup:mainfrom Dec 12, 2025
Merged
Introduce KHR_external_semaphore_fd only for FD==-1#2670billhollings merged 1 commit intoKhronosGroup:mainfrom
billhollings merged 1 commit intoKhronosGroup:mainfrom
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There is a special use case for when importing/exporting a semaphore of handle type `VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT` and FD == -1. According to the Vulkan specs, calling `vkImportSemaphoreFdKHR` has the effect of signaling the binary semaphore and calling `vkGetSemaphoreFdKHR` has the effect of waiting on the binary semaphore. Outside of KHR_external_semaphore_fd extension, there is no other way to achieve this behaviour. For example, `vkWaitSemaphores` does not operate on a binary semaphore and there is no other defined way for a CPU to send a signal operation. In theory, we can use EXT_metal_objects and export the VkSemaphore to a `MTLSharedEvent` but we do not have access to the `MVKSemaphoreMTLEvent` in order to determine the `_mtlEventValue` (last encoded wait value). This functionality is required to implement Venus on macOS which uses these functions to synchronize the guest semaphore state to the host.
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Thanks for submitting! LGTM.
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There is a special use case for when importing/exporting a semaphore of handle type
VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BITand FD == -1. According to the Vulkan specs, callingvkImportSemaphoreFdKHRhas the effect of signaling the binary semaphore and callingvkGetSemaphoreFdKHRhas the effect of waiting on the binary semaphore.Outside of KHR_external_semaphore_fd extension, there is no other way to achieve this behaviour. For example,
vkWaitSemaphoresdoes not operate on a binary semaphore and there is no other defined way for a CPU to send a signal operation.In theory, we can use EXT_metal_objects and export the VkSemaphore to a
MTLSharedEventbut we do not have access to theMVKSemaphoreMTLEventin order to determine the_mtlEventValue(last encoded wait value).This functionality is required to implement Venus on macOS which uses these functions to synchronize the guest semaphore state to the host.