VK_EXT_map_memory_placed

This document proposes adding support for application-controlled virtual address placement of VkDeviceMemory maps.

1. Problem Statement

There are certain cases in which it can be useful to directly control the address used by a memory map operation. One example of this is in emulation environments when the application under emulation is 32-bit but the host userspace is 64-bit. In order to avoid additional copies or address translation, the emulator can choose an address in the lower 32 bits of the address space and request that the map be placed there.

This functionality is already supported by the standard UNIX mmap() system call which provides an address hint flag as its first parameter as well as a MAP_FIXED flag which causes the mmap() to fail if it cannot place the map in exactly that location. This extension proposes to add similar functionality to vkMapMemory().

2. Solution Space

It is possible to emulate this functionality using VK_EXT_external_memory_host by creating a placed memory mapping in some other way (such as a Linux memfd combined with conventional mmap()) and then importing the resulting map via VK_EXT_external_memory_host. However, this requires every mappable Vulkan memory allocation to be a host pointer import which places restrictions available memory types and heaps and may have significant performance costs.

One alternative approach to the emulation use-case would be to add a VK_MEMORY_MAP_32BIT_ADDRESS_BIT_EXT which does exactly what emulation wants and places the map in the lower 32 bits. This is equivalent to the Linux MAP_32BIT flag. However, this is less flexible and may be difficult to implement on Arm platforms where Linux mmap() does not support MAP_32BIT since drivers are likely relying on mmap() to implement vkMapMemory().

3. Proposal

3.1. Dependencies

  • VK_EXT_map_memory2

3.2. API Features

The following features are exposed by this extension:

typedef struct VkPhysicalDeviceMapMemoryPlacedFeaturesEXT {
    VkStructureType    sType;
    void*              pNext;
    VkBool32           memoryMapPlaced;
    VkBool32           memoryMapRangePlaced;
    VkBool32           memoryUnmapReserve;
} VkPhysicalDeviceMapMemoryPlacedFeaturesEXT;

typedef struct VkPhysicalDeviceMapMemoryPlacedPropertiesEXT {
    VkStructureType    sType;
    void*              pNext;
    VkDeviceSize       minPlacedMemoryMapAlignment;
} VkPhysicalDeviceMapMemoryPlacedPropertiesEXT;

typedef enum VkMemoryMapFlagBits {
    VK_MEMORY_MAP_PLACED_BIT_EXT = 0x00000001,
} VkMemoryMapFlagBits;

typedef struct VkMemoryMapPlacedInfoEXT {
    VkStructureType    sType;
    const void*        pNext;
    void*              pPlacedAddress;
} VkMemoryMapPlacedInfoEXT;

typedef enum VkMemoryUnmapFlagBitsKHR {
    VK_MEMORY_UNMAP_RESERVE_BIT_EXT = 0x00000001,
} VkMemoryMapFlagBitsKHR;

  1. memoryMapPlaced indicates support for placed memory maps

  2. memoryMapRangePlaced indicates support for placed memory maps of subranges of the VkDeviceMemory object.

  3. memoryUnmapReserve indicates support for VK_MEMORY_UNMAP_RESERVE_BIT_EXT.

When VK_MEMORY_MAP_PLACED_BIT_EXT is set in the flags member of VkMemoryMapInfoKHR and a VkMapMemoryPlacedInfoEXT structure is included in the pNext chain of VkMemoryMapInfoKHR, the implementation attempts to place the map at the address specified by VkMapMemoryPlacedInfoEXT::pPlacedAddress. If the map cannot be placed exactly at the specified virtual address, vkMapMemory2KHR() returns VK_ERROR_MEMORY_MAP_FAILED.

When VK_MEMORY_UNMAP_RESERVE_BIT_EXT is set in the flags member of VkMemoryUnmapInfoEXT, the memory object will be unmapped but the address range will remain reserved so that another call to mmap() or vkMapMemory() will not reuse that address range without an address hint or VK_MEMORY_MAP_PLACED_BIT_EXT, respectively.

4. Examples

The following example allocates a memory object and maps it in the lower 32 bit address space, using mmap() to select the address.

VkDeviceMemory mem;
const VkMemoryAllocateInfo allocInfo = {
    .sType = VK_STRUCTURE_TYPE_MEMORY_ALLOCATE_INFO,
   .allocationSize = /* compute size */,
   .memoryTypeIndex = /* compute memory type */,
};
VK_CHECK(vkAllocateMemory(device, &allocInfo, NULL, &mem));

void *reserved = mmap(NULL, allocInfo.allocationSize, PROT_NONE,
                      MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1, 0);
CHECK(reserved != MAP_FAILED);

const VkMemoryMapPlacedInfoEXT mapPlacedInfo = {
    .sType = VK_STRUCTURE_TYPE_MEMORY_MAP_PLACED_INFO_EXT,
    .pPlacedAddress = reserved,
};
const VkMemoryMapInfoKHR mapInfo = {
    .sType = VK_STRUCTURE_TYPE_MEMORY_MAP_INFO_KHR,
    .pNext = &mapPlacedInfo,
    .memory = mem,
    .offset = 0,
    .size = VK_WHOLE_SIZE,
    .flags = VK_MEMORY_MAP_PLACED_BIT_EXT,
};
void *map;
VK_CHECK(vkMapMemory2KHR(device, &mapInfo, &map));
CHECK(map == reserved);

5. Issues

1) How should the requested address be passed to vkMapMemory2KHR()?

RESOLVED: When VK_MEMORY_MAP_PLACED_BIT_EXT is passed to vkMapMemory2KHR() and a VkMemoryMapPlacedInfoEXT structure is present in the pNext chain, the implementation attempts to map to whatever address is provided by VkMemoryMapPlacedInfoEXT::pPlacedAddress.

2) What should happen if the implementation cannot place the memory map at the requested address?

RESOLVED: The memory object should be left unmapped and vkMapMemory2KHR should return VK_ERROR_MEMORY_MAP_FAILED.

3) How can an application atomically re-reserve the address range on unmap?

RESOLVED: When VK_MEMORY_UNMAP_RESERVE_BIT_EXT is passed to vkUnmapMemory2KHR(), the implementation unmaps the memory range in such a way that the range is automatically re-reserved. With mmap(), this is accomplished by simply mapping over the range with another anonymous mapping. However, allowing this with vkMapMemory() would break Vulkan’s concept of when a memory object is or is not mapped.

4) Should a placed memory map replace existing maps in the specified range or fail if a map exists?

RESOLVED: It should replace existing maps. If an application wants try-map behavior, it can get that by using mmap with MAP_ANONYMOUS and an address and only call vkMapMemory2KHR() to do a placed map if that succeeds. Without replacement, there is no way for the application to atomically exchange maps and prevent races between reservation and the final map. Also, any attempt at a middle ground in which it might replace would make the spec less well-defined.