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

Hi guys,

one theoretical alternative not mentioned in this thread is the use of 
mremap().

In other words you reserve some address space below 2G by using 
mmap(NULL, length, PROT_NONE, MAP_32BIT | MAP_ANONYMOUS, 0, 0) and then 
use mremap(addr64bit, 0, length, MREMAP_FIXED, reserved_addr).

I haven't tested this but at least in theory it should give you a 
duplicate of the 64bit mapping in the lower 2G of the address space.

Important is that you give 0 as oldsize to mremap() so that the old 
mapping isn't unmapped but rather just a new mapping of the existing VMA 
created.

Regards,
Christian.


Am 18.10.24 um 23:55 schrieb 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
>