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
