Hi Derek,
Am 31.10.24 um 11:05 schrieb Derek Lesho:
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?
Good question, I'm not 100% sure. I would need to double check with
upstream on that, but before I would do this I would first like to know
if it's really a doable option.
- 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?
Private anonymous means that you *can't* map the memory at two different
locations because it is actually private and writing into it from a
second mapping would trigger copy on write.
The name of the MREMAP_DONTUNMAP flag is also really a misleading.
Instead of "not unmapping" what happens is that you atomically move a
private anonymous mapping from A to B and at the same time create a new
mapping at A with zeroed memory. Not really sure what that is good for.
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.
To be honest I can't say what's the right approach either. I've seen the
mremap() approach being used on emulators before and it's usually really
useful there.
Just wanted to point out that nobody has mentioned the mremap()
functionality in the discussion and then became curious why it works for
files and shared memory but doesn't for GPU driver mappings :)
Regarding an OpenGL or Vulkan extension based approach it's really
tricky to do this as well. Basically only the driver knows what and how
data needs to be mapped and only Wine knows were it can be mapped. So
you need to exchange a bunch of information like size and alignment of a
mapping to actually make it work.
Regards,
Christian.
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
