No way to get a device address aligned to a spec-required alignment without overallocating

[Cyphall]

vkAllocateMemory says:

Allocations returned by vkAllocateMemory are guaranteed to meet any alignment requirement of the implementation.

But in some cases, the alignments are required by the spec itself and happen to be higher than this maximum implementation alignment requirement.

An example of this is:

VUID-VkAccelerationStructureCreateInfo2KHR-addressRange-11605
addressRange.address must be a multiple of 256 bytes

If the implementation doesn't require a min alignment >= 256 for anything, the allocated memory may not be aligned to 256 bytes.
This means a simple alignment of the offset within the memory is no longer sufficient.

Currently, the only way to avoid this problem is to overallocate by alignment - 1 and manually pad the device address.

[HansKristian-Work]

I think we should be able to guarantee that at least if RT is supported.

[Cyphall]

Yes, ideally vkAllocateMemory should be aligned to max(maxImplemAlignment, maxEnabledFeatureSpecAlignment) where maxImplemAlignment is the existing requirement and maxEnabledFeatureSpecAlignment is the max alignment required by the spec for enabled features.

[HansKristian-Work]

Yes, that's roughly what I will propose as a spec fix.

[Cyphall]

vkAllocateMemory should be aligned to max(maxImplemAlignment, maxEnabledFeatureSpecAlignment)

Actually, maxSupportedFeatureSpecAlignment instead of maxEnabledFeatureSpecAlignment, otherwise a VkDevice with feature disabled could share an underaligned alloc with a VkDevice with feature enabled.

[Cyphall]

Looks like this is related to #2794

[HansKristian-Work]

Yes, it's a similar issue.