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

Fri Oct 18 22:47:19 UTC 2024

The timing here isn't great, unfortunately. I'd love to contribute more to
the discussion but I'm going on leave starting next week until mid-Febuary
so I won't be able to participate much until then. I'll try to leave a few
thoughts, though.

On Fri, Oct 18, 2024 at 5:10 PM Derek Lesho <dlesho at codeweavers.com> wrote:

> 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.
>

It's an interesting approach, to be sure. I don't mean that as a bad or
good thing as I haven't given this enough thought with GL in mind to have a
better, more well thought out plan.

The most obvious issue that jumps out to me is that we really want that
callback to be set before anyone ever maps a buffer that might possibly get
exposed to the client and we want it to never change.  If this were Vulkan,
we'd have you provide it at vkCreateDevice() time.  But this is GL where
everybody loves a big mutable state object. If we do go with callbacks (and
it's still not 100% clear to me what the right choice is), we'd want them
to be somehow set-once and set before any buffers are created.  I'm not
100% sure how you'd spec that or how we'd enforce it.  There may be some
precedent for this somewhere in GL (no_error, maybe?) but I'm not sure.

The other question that comes to mind is when exactly we'd be expected to
use these things. Obviously, we need to do so for any map that may be
exposed to the client.  However, it's not always easy to do that because
you don't know at buffer create time whether or not it will be persistently
mapped.  A driver is likely to have all sorts of internal mappings for
things and, while those can come from one of those ranges, it'll burn more
of that precious 32-bit address space than needed. This gets worse when you
take sub-allocation into account. If we're okay with all buffer mappings
going down the client-request path then it's probably okay. The driver just
might need an extra bit in its buffer cache key.

I'm also sitting here trying to come up with some plan that would let us do
this more like Vulkan and I'm having trouble coming up with one that works.
GL has no concept of "create time". We could theoretically do something
where we flush everything, copy the data to a new placed-mappable buffer
and then continue on but that's gonna suck.

I think that's all that comes to mind immediately. As I said at the top,
I'm happy to talk more in a few months. Best of luck until then!

~Faith

> 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.
>

For cases where Zink is being used on the host (this is the current plan
for Nouveau going forward), doing Zink in Windows may not be a bad idea.
However, I agree that it may not be the best idea to rely on that plan.

> 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
>
>
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