Hi Christian,
Thanks for exploring this option, it's great to have a better understanding of the capabilities/limits of mremap.
First, a few practical questions:
- How would Wine detect the updated mremap, I suppose we would want to create a test persistent mapping on init and see if mremap works on that page?
- In my experience Mesa will sometimes return malloc'd pointers from glMapBuffer when using the transfer helper, which I assume has to do with buffer textures. If wine blindly uses remap for all glMapBuffer calls, we'll then end up hitting the private anonymous path, where from what I understand page faults are use to preserve the old page, potentially slowing things down, thoughts?
Second, in regards to going forward:
I don't have the final call on what path we end up taking 😅. I think it would be good if others from wine-devel pitched into what they think, but here's my opinion:
I think the mremap approach should definitely be pursued since it looks like such a simple Kernel patch, but it may also be good to pursue a simple Mesa-Integrated path as well, maybe with Mesa calling into a Wine library to allocate 32-bit pages, as we support non-linux OS's as well.
Thanks, Derek
Am 10/30/24 um 14:03 schrieb Christian König:
Hi guys,
so I looked a bit deeper into the problem of duplicating graphics driver mappings with mremap().
This use case of duplicating a mapping into a fixed address is already supported quite well using mremap(). This is used by a couple of different emulators to re-create the address space like you would find it in the specific environment.
The only problem is that this only works for files and shared memory at the moment. Graphic driver mappings on the other hand have the VM_DONTEXPAND and VM_PFNMAP flag set because their mappings shouldn't grow and can also include VRAM.
The attached patch changes this restriction for the mremap() function and so also allows duplicating the VMAs of graphics drivers into the lower 32bit address space managed by Wine.
I've tested this with some of AMD's GPU unit tests and it actually seems to work quite fine.
Derek please let me know if that solution works for you and if you're interested in using it. If yes I would go ahead and send the patch to the Linux memory management folks for discussion.
Regards, Christian.
Am 24.10.24 um 17:06 schrieb Christian König:
Darek we are unfortunately both partially right.
Linux supports cloning VMAs using mremap() from userspace by using a zero old size, but unfortunately only for SHM areas.
See the code in mm/mremap.c:        /*         * We allow a zero old-len as a special case         * for DOS-emu "duplicate shm area" thing. But         * a zero new-len is nonsensical.         */        if (!new_len)                return ret;
Going to take a closer look to figure out what would be necessary to solve that for GPU drivers as well.
Regards, Christian.
Am 24.10.24 um 14:56 schrieb Christian König:
I haven't tested it but as far as I know that isn't correct.
As far as I know you can map the same VMA at a different location even without MREMAP_DONTUNMAP. And yes MREMAP_DONTUNMAP only work with private mappings, but that isn't needed here.
Give me a moment to test this.
Regards, Christian.
Am 24.10.24 um 10:03 schrieb Derek Lesho:
In my last mail I responded to this approach all the way at the bottom, so it probably got lost: mremap on Linux as it exists now won't work as it only supports private anonymous mappings (in conjunction with MREMAP_DONTUNMAP), which GPU mappings are not.
Am 10/24/24 um 01:06 schrieb James Jones:
That makes sense. Reading the man page myself, it does seem like:
-If the drivers can guarantee they set MAP_SHARED when creating their initial mapping.
-If WINE is fine rounding down to page boundaries to deal with mappings of suballocations and either using some lookup structure to avoid duplicate remappings (probably needed to handle unmap anyway per below) or just living with the perf cost and address space overconsumption for duplicate remappings.
-If mremap() preserves the cache attributes of the original mapping.
Then no GL API change would be needed. WINE would just have to do an if (addrAbove4G) { mremapStuff() } on map and presumably add some tracking to perform an equivalent munmap() when unmapping. I assume WINE already has a bunch of vaddr tracking logic in use to manage the <4G address space as described elsewhere in the thread. That would be pretty ideal from a driver vendor perspective.
Does that work?
Thanks, -James
On 10/23/24 06:12, Christian König wrote:
I haven't read through the whole mail thread, but if you manage the address space using mmap() then you always run into this issue.
If you manage the whole 4GiB address space by Wine then you never run into this issue. You would just allocate some address range internally and mremap() into that.
Regards, Christian.
Am 22.10.24 um 19:32 schrieb James Jones: > This sounds interesting, but does it come with the same "Only > gets 2GB VA" downside Derek pointed out in the thread fork where > he was responding to Michel? > > Thanks, > -James > > On 10/22/24 07:14, Christian König wrote: >> 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 >>> >>
-
Derek Lesho