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userspace: introduce coconut-alloc as allocator#927

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n-ramacciotti:feature/userspace_allocator
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userspace: introduce coconut-alloc as allocator#927
n-ramacciotti wants to merge 17 commits into
coconut-svsm:mainfrom
n-ramacciotti:feature/userspace_allocator

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@n-ramacciotti n-ramacciotti commented Dec 19, 2025 •
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This PR introduces heap allocation support in userspace.

The heap is a single 64KiB static block initialized by userlib at startup before calling the main function. Atomic state operations ensure that only a single initialization can occur.
The PR implements the GlobalAlloc trait by exploiting AllocBlock from coconut-alloc, which supports lockless allocations up to 32KiB using atomic operations.

This PR is marked as draft as it depends on the changes from:

  • #4 from coconut-alloc: this removes an unstable feature not available in the current toolchain (1.86.0) and adds a new utility function required in this PR
  • #937: this updates rust toolchain to 1.87.0 as required by coconut-alloc

These changes were originally included in #913, which also introduced test in userspace svsm, but as suggested, I split them into multiple PRs.

All feedback and suggestions are welcome

@n-ramacciotti n-ramacciotti mentioned this pull request Dec 19, 2025
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@stefano-garzarella stefano-garzarella added the needs-rebase The PR needs to be rebased to the latest upstream branch label Dec 19, 2025
@n-ramacciotti n-ramacciotti force-pushed the feature/userspace_allocator branch from 69d697d to 4a8ed09 Compare December 19, 2025 18:28
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n-ramacciotti commented Dec 19, 2025

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v2:

  • rebased on latest main

@peterfang peterfang added TSC review pending This PR requires discussion among the TSC before it can be finalized and removed needs-rebase The PR needs to be rebased to the latest upstream branch labels Dec 22, 2025
@peterfang peterfang removed the TSC review pending This PR requires discussion among the TSC before it can be finalized label Jan 6, 2026
@n-ramacciotti n-ramacciotti force-pushed the feature/userspace_allocator branch from 4a8ed09 to 2a36887 Compare January 9, 2026 18:13
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n-ramacciotti commented Jan 9, 2026

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v3:

@n-ramacciotti n-ramacciotti force-pushed the feature/userspace_allocator branch from 2a36887 to 622b9b2 Compare January 12, 2026 14:53
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n-ramacciotti commented Jan 12, 2026

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v4:

  • Rebased on latest main, which includes rust 1.87.0

@joergroedel joergroedel self-assigned this Jan 12, 2026
joergroedel
joergroedel reviewed Jan 12, 2026
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Thanks for your contribution, this looks very promising and I am happy to see progress on the user-mode allocator implementation. Left some comments to consider.

Comment thread user/lib/src/alloc.rs Outdated
Comment thread user/lib/src/alloc.rs Outdated
Comment thread user/lib/src/alloc.rs Outdated
Comment thread user/lib/src/alloc.rs
@joergroedel joergroedel added the in-review PR is under active review and not yet approved label Jan 12, 2026
joergroedel and others added 16 commits January 22, 2026 17:06
@joergroedel @n-ramacciotti
syscall: Add MMAP/MUNMAP/MRESIZE system call wrappers
2eff6bd 
Add the wrappers for system calls which change the memory layout of
the user-mode task.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Add conversion from MMFlags to VMFileMappingFlags
adf32b5 
Add conversion support for the user-mode representation of memory
mapping flags to the kernel representation.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/fs: Get Handles from struct FsObj
1c56071 
Implement methods to retrieve a reference to the FileHandle or
DirectoryHandle the object points to.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/syscall: Provide MMAP system call
06ac1aa 
Implement the system call to allow user-mode applications to change
their memory layout.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/syscall: Provide MUNMAP system call
6cb0d47 
Implement the system call to unmap memory mappings from the virtual
address space.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Add resize() and flush() methods to trait VirtualMapping
3511af0 
These two methods can be implemented for trait VirtualMapping to
support resizing of an existing mapping.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Make VMR::map/unmap_vmm() methods more generic
76528e5 
Convert the map_vmm() and unma_vmm() methods to the more generic
map/unmap_range() methods which can handle sub-ranges of VMMs as well.
Re-implement map/unmap_vmm() to use the more generic methods.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Move VMR TLB flush logic to helper function
793eaeb 
Move the TLB flush logic to a helper function to re-use it at other
places.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Implement VMM resize operation
7b686e7 
Add a method to struct VMR to resize an existing mapping.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@joergroedel @n-ramacciotti
kernel/mm: Implement resize support for struct VMalloc
820fa7c 
Support the resizing of VMalloc mappings.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
@n-ramacciotti
[hack] kernel/task: Change console to obj handle 1
047429e 
This is a workaround to use mmap for anonymous mapping.
Currently, mmap looks for obj identifier to identify if
the mapping is anonymous or not. Anonymous mappings for
the moment is based on the fact that id 0 is free, but
it is utilized by the console.

This commit fix this by changing the identifier to 1
and it will be removed when mmap will be updated. It
has been introduced to test the rest of the allocator
logic based on mmap.

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/lib: Introduce coconut-alloc in userlib
b3aa652 
Add dependency for coconut-alloc crate in the workspace

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/lib: Introduce AllocBlock from coconut-alloc
5d32d88 
Introduce a 64 KiB static heap using `AllocBlock` to support dynamic
memory allocation in userspace.

Initialize the global heap in the `init()` function from `userlib` before
calling the `main` function of the crate. Ensure single initialization of
the heap by using `OnceRef`.

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/lib: Implement GlobalAlloc trait to use the alloc crate
61dbc29 
Implement the `GlobalAlloc` trait by wrapping the lockless `alloc`
and `free` operations of `AllocBlock`, which use atomic operations
internally. Since those operation return offset from the start of
the block, convert them to pointers relative to the heap base.

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/lib: Implement utilities to extract Layout from size or pointer
dfb9bf6 
Implement `layout_from_size()` and `layout_from_ptr()` utilities to
construct `Layout.

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/lib: Add allocator unit tests
9c1b693 
Disable custom global allocator during normal test execution
since the default allocator is sufficient for unit testing

To avoid unused warnings in test builds, move `SvsmUserAllocator`
implementation behind a configuration flag

Add unit tests for layout calculation.

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti
user/init: Add heap allocation example using alloc crate
0b4a6eb 
Allocate a Vec to show that the verify that the userspace
memory allocator works correctly

Signed-off-by: Nicola Ramacciotti <niko.ramak@gmail.com>
@n-ramacciotti n-ramacciotti force-pushed the feature/userspace_allocator branch from 622b9b2 to 0b4a6eb Compare January 22, 2026 17:55
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n-ramacciotti commented Jan 22, 2026

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v5:

  • Rebased on latest main, which includes rust 2024 version.
  • Included mmap PR to use mmap syscall.
  • Added commit 047429e as a temporary workaround for the problem of anonymous mapping. More details are in the commit description.
  • Addressed @joergroedel's comments:
    • use mmap for the heap area
    • panic when trying to access a non initialized HEAP
    • remove unnecessary println
    • use OnceRef for single initalization of the heap
  • With these new changes the test function for alloc and dealloc had to be removed since now it is required an online usage.

TODO:

  • implement get_global_heap and set_global_heap on a new AllocBlock available when testing. So it is possible to test alloc and dealloc of the GlobalAlloc trait.

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