Helping Wine use 64 bit Mesa OGL drivers for 32-bit Windows applications

[Derek Lesho]

Hey everyone 👋,

I'm Derek from the Wine project, and wanted to start a discussion with 
y'all about potentially extending the Mesa OGL drivers to help us with a 
functionality gap we're facing.

Problem Space:

In the last few years Wine's support for running 32-bit windows apps in 
a 64-bit host environment (wow64) has almost reached feature completion, 
but there remains a pain point with OpenGL applications: Namely that 
Wine can't return a 64-bit GL implementation's buffer mappings to a 32 
bit application when the address is outside of the 32-bit range.

Currently, we have a workaround that will copy any changes to the 
mapping back to the host upon glBufferUnmap, but this of course is slow 
when the implementation directly returns mapped memory, and doesn't work 
for GL_PERSISTENT_BIT, where directly mapped memory is required.

A few years ago we also faced this problem with Vulkan's, which was 
solved through the VK_EXT_map_memory_placed extension Faith drafted, 
allowing us to use our Wine-internal allocator to provide the pages the 
driver maps to. I'm now wondering if an GL equivalent would also be seen 
as feasible amongst the devs here.

Proposed solution:

As the GL backend handles host mapping in its own code, only giving 
suballocations from its mappings back to the App, the problem is a 
little bit less straight forward in comparison to our Vulkan solution: 
If we just allowed the application to set its own placed mapping when 
calling glMapBuffer, the driver might then have to handle moving buffers 
out of already mapped ranges, and would lose control over its own memory 
management schemes.

Therefore, I propose a GL extension that allows the GL client to provide 
a mapping and unmapping callback to the implementation, to be used 
whenever the driver needs to perform such operations. This way the 
driver remains in full control of its memory management affairs, and the 
amount of work for an implementation as well as potential for bugs is 
kept minimal. I've written a draft implementation in Zink using 
map_memory_placed [1] and a corresponding Wine MR utilizing it [2], and 
would be curious to hear your thoughts. I don't have experience in the 
Mesa codebase, so I apologize if the branch is a tad messy.

In theory, the only requirement from drivers from the extension would be 
that glMapBuffer always return a pointer from within a page allocated 
through the provided callbacks, so that it can be guaranteed to be 
positioned within the required address space. Wine would then use it's 
existing workaround for other types of buffers, but as Mesa seems to 
often return directly mapped buffers in other cases as well, Wine could 
also avoid the slowdown that comes with copying in these cases as well.

Why not use Zink?:

There's also a proposal to use a 32-bit PE build of Zink in Wine 
bypassing the need for an extension; I brought this to discussion in 
this Wine-Devel thread last week [3], which has some arguments against 
this approach.


If any of you have thoughts, concerns, or questions about this potential 
approach, please let me know, thanks!

1: https://gitlab.freedesktop.org/Guy1524/mesa/-/commits/placed_allocation

2: https://gitlab.winehq.org/wine/wine/-/merge_requests/6663

3: https://marc.info/?t=172883260300002&r=1&w=2