Requires Vulkan 1.0
Promoted to Vulkan 1.1
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- Interactions and External Dependencies
Jason Ekstrand, Intel
Ian Elliot, Google
Jesse Hall, Google
Tobias Hector, Imagination Technologies
James Jones, NVIDIA
Jeff Juliano, NVIDIA
Matthew Netsch, Qualcomm Technologies, Inc.
Daniel Rakos, AMD
Carsten Rohde, NVIDIA
Ray Smith, ARM
Chad Versace, Google
An application may wish to reference device memory in multiple Vulkan logical devices or instances, in multiple processes, and/or in multiple APIs. This extension enables an application to export non-Vulkan handles from Vulkan memory objects such that the underlying resources can be referenced outside the scope of the Vulkan logical device that created them.
All functionality in this extension is included in core Vulkan 1.1, with the KHR suffix omitted. The original type, enum and command names are still available as aliases of the core functionality.
1) How do applications correlate two physical devices across process or Vulkan instance boundaries?
RESOLVED: New device ID fields have been introduced by
These fields, combined with the existing
driverVersion field can be used to
identify compatible devices across processes, drivers, and APIs.
pipelineCacheUUID is not sufficient
for this purpose because despite its description in the specification, it
need only identify a unique pipeline cache format in practice.
Multiple devices may be able to use the same pipeline cache data, and hence
it would be desirable for all of them to have the same pipeline cache UUID.
However, only the same concrete physical device can be used when sharing
memory, so an actual unique device ID was introduced.
Further, the pipeline cache UUID was specific to Vulkan, but correlation
with other, non-extensible APIs is required to enable interoperation with
2) If memory objects are shared between processes and APIs, is this considered aliasing according to the rules outlined in the Memory Aliasing section?
RESOLVED: Yes. Applications must take care to obey all restrictions imposed on aliased resources when using memory across multiple Vulkan instances or other APIs.
3) Are new image layouts or metadata required to specify image layouts and layout transitions compatible with non-Vulkan APIs, or with other instances of the same Vulkan driver?
RESOLVED: Separate instances of the same Vulkan driver running on the same GPU should have identical internal layout semantics, so applications have the tools they need to ensure views of images are consistent between the two instances. Other APIs will fall into two categories: Those that are Vulkan- compatible, and those that are Vulkan-incompatible. Vulkan-incompatible APIs will require the image to be in the GENERAL layout whenever they are accessing them.
Note this does not attempt to address cross-device transitions, nor transitions to engines on the same device which are not visible within the Vulkan API. Both of these are beyond the scope of this extension.
4) Is a new barrier flag or operation of some type needed to prepare external memory for handoff to another Vulkan instance or API and/or receive it from another instance or API?
RESOLVED: Yes. Some implementations need to perform additional cache management when transitioning memory between address spaces, and other APIs, instances, or processes may operate in a separate address space. Options for defining this transition include:
A new structure has the advantage that the type of external transition can
be described in as much detail as necessary.
However, there is not currently a known need for anything beyond
differentiating external vs.
internal accesses, so this is likely an over-engineered solution.
The access flag bit has the advantage that it can be applied at buffer,
image, or global granularity, and semantically it maps pretty well to the
operation being described.
Additionally, the API already includes
VK_ACCESS_MEMORY_WRITE_BIT which appear to be intended for this
However, there is no obvious pipeline stage that would correspond to an
external access, and therefore no clear way to use
VkDependencyFlags and VkPipelineStageFlags operate at command
granularity rather than image or buffer granularity, which would make an
entire pipeline barrier an internal→external or external→internal barrier.
This may not be a problem in practice, but seems like the wrong scope.
Another downside of VkDependencyFlags is that it lacks inherent
directionality: There are not
dst variants of it in the
barrier or dependency description semantics, so two bits might need to be
added to describe both internal→external and external→internal
Transitioning a resource to a special queue family corresponds well with the
operation of transitioning to a separate Vulkan instance, in that both
operations ideally include scheduling a barrier on both sides of the
transition: Both the releasing and the acquiring queue or process.
Using a special queue family requires adding an additional reserved queue
VK_QUEUE_FAMILY_IGNORED would have left it unclear how to
transition a concurrent usage resource from one process to another, since
the semantics would have likely been equivalent to the currently-ignored
Fortunately, creating a new reserved queue family index is not invasive.
Based on the above analysis, the approach of transitioning to a special “external” queue family was chosen.
5) Do internal driver memory arrangements and/or other internal driver image properties need to be exported and imported when sharing images across processes or APIs.
RESOLVED: Some vendors claim this is necessary on their implementations, but it was determined that the security risks of allowing opaque meta data to be passed from applications to the driver were too high. Therefore, implementations which require metadata will need to associate it with the objects represented by the external handles, and rely on the dedicated allocation mechanism to associate the exported and imported memory objects with a single image or buffer.
6) Most prior interoperation and cross-process sharing APIs have been based on image-level sharing. Should Vulkan sharing be based on memory-object sharing or image sharing?
RESOLVED: These extensions have assumed memory-level sharing is the correct granularity. Vulkan is a lower-level API than most prior APIs, and as such attempts to closely align with to the underlying primitives of the hardware and system-level drivers it abstracts. In general, the resource that holds the backing store for both images and buffers of various types is memory. Images and buffers are merely metadata containing brief descriptions of the layout of bits within that memory.
Because memory object-based sharing is aligned with the overall Vulkan API design, it exposes the full power of Vulkan on external objects. External memory can be used as backing for sparse images, for example, whereas such usage would be awkward at best with a sharing mechanism based on higher-level primitives such as images. Further, aligning the mechanism with the API in this way provides some hope of trivial compatibility with future API enhancements. If new objects backed by memory objects are added to the API, they too can be used across processes with minimal additions to the base external memory APIs.
Earlier APIs implemented interop at a higher level, and this necessitated entirely separate sharing APIs for images and buffers. To co-exist and interoperate with those APIs, the Vulkan external sharing mechanism must accommodate their model. However, if it can be agreed that memory-based sharing is the more desirable and forward-looking design, legacy interoperation considerations can be considered another reason to favor memory-based sharing: While native and legacy driver primitives that may be used to implement sharing may not be as low-level as the API here suggests, raw memory is still the least common denominator among the types. Image-based sharing can be cleanly derived from a set of base memory- object sharing APIs with minimal effort, whereas image-based sharing does not generalize well to buffer or raw-memory sharing. Therefore, following the general Vulkan design principle of minimalism, it is better to expose even interopability with image-based native and external primitives via the memory sharing API, and place sufficient limits on their usage to ensure they can be used only as backing for equivalent Vulkan images. This provides a consistent API for applications regardless of which platform or external API they are targeting, which makes development of multi-API and multi-platform applications simpler.
7) Should Vulkan define a common external handle type and provide Vulkan functions to facilitate cross-process sharing of such handles rather than relying on native handles to define the external objects?
RESOLVED: No. Cross-process sharing of resources is best left to native platforms. There are myriad security and extensibility issues with such a mechanism, and attempting to re-solve all those issues within Vulkan does not align with Vulkan’s purpose as a graphics API. If desired, such a mechanism could be built as a layer or helper library on top of the opaque native handle defined in this family of extensions.
8) Must implementations provide additional guarantees about state implicitly included in memory objects for those memory objects that may be exported?
RESOLVED: Implementations must ensure that sharing memory objects does not transfer any information between the exporting and importing instances and APIs other than that required to share the data contained in the memory objects explicitly shared. As specific examples, data from previously freed memory objects that used the same underlying physical memory, and data from memory obects using adjacent physical memory must not be visible to applications importing an exported memory object.
9) Must implementations validate external handles the application provides as input to memory import operations?
RESOLVED: Implementations must return an error to the application if the provided memory handle cannot be used to complete the requested import operation. However, implementations need not validate handles are of the exact type specified by the application.
For more information, see the Vulkan Specification
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Copyright (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.