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PORTING.md

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Porting snmalloc to a new platform

All of the platform-specific logic in snmalloc is isolated in the Platform Abstraction Layer (PAL). To add support for a new platform, you will need to implement a new PAL for your system.

After version 0.5.2, the PAL is defined by a C++20 Concept. When compiling with C++20 or later, you should get helpful messages about any fields or methods that your PAL is missing.

The PAL must implement the following methods:

[[noreturn]] static void error(const char* const str) noexcept;

Report a fatal error and exit.

The memory that snmalloc is supplied from the Pal should be in one of three states

  • using
  • using readonly
  • not using

Before accessing the memory for a read, snmalloc will change the state to either using or using readonly, and before a write by it will change the state to using. If memory is not required any more, then snmalloc will change the state to not using, and will ensure that it notifies the Pal again before it every accesses that memory again.
The not using state allows the Pal to recycle the memory for other purposes. If pal_enforce_access is set as a mitigation, then accessing memory that has not been notified correctly should trigger an exception/segfault.

The state for a particular region of memory is set with

static void notify_not_using(void* p, size_t size) noexcept;

template<ZeroMem zero_mem>
static void notify_using(void* p, size_t size) noexcept;

static void notify_using_readonly(void* p, size_t size) noexcept;

These functions notify the system that the range of memory from p to p + size is in the relevant state.

If the template parameter is set to YesZero then notify_using must ensure the range is full of zeros.

template<bool page_aligned = false>
static void zero(void* p, size_t size) noexcept;

Zero the range of memory from p to p + size. This may be a simple memset call, but the page_aligned template parameter allows for more efficient implementations when entire pages are being zeroed. This function is typically called with very large ranges, so it may be more efficient to request that the operating system provides background-zeroed pages, rather than zeroing them synchronously in this call

template<bool state_using>
static void* reserve_aligned(size_t size) noexcept;
static void* reserve(size_t size) noexcept;

All platforms should provide reserve and can optionally provide reserve_aligned if the underlying system can provide strongly aligned memory regions. If the system guarantees only page alignment, implement only the second. snmalloc will overallocate to ensure it can find suitably aligned blocks inside the region. reserve should consider memory initially as not_using, and snmalloc will notify when it needs the range of memory.

If the system provides strong alignment, implement the first to return memory at the desired alignment. If providing the first, then the Pal should also specify the minimum size block it can provide:

static constexpr size_t minimum_alloc_size = ...;

The PAL is also responsible for advertising the page size:

static constexpr size_t page_size = 0x1000;

This is the granularity at which the PAL is able to mark memory as in-use or not-in-use. The PAL is free to advertise a size greater than the minimum page size if that would be more efficient. When a slab is deallocated, the PAL will be instructed to mark everything after the first page_size bytes as not-in-use and so larger values can lead to more memory overhead.

The page size for any given system depends on both the underlying architecture and the operating system. The value exposed by the PAL may also depend on the Architecture Abstraction Layer (AAL). For example, the Linux PAL advertises 64 KiB on PowerPC but 4 KiB on every other supported architecture:

static constexpr size_t page_size =
	Aal::aal_name == PowerPC ? 0x10000 : 0x1000;

Finally, you need to define a field to indicate the features that your PAL supports:

static constexpr uint64_t pal_features = ...;

These features are defined in the PalFeatures enumeration.

There are several partial PALs that can be used when implementing POSIX-like systems:

  • PALPOSIX defines a PAL for a POSIX platform using no non-standard features.
  • PALBSD defines a PAL for the common set of BSD extensions to POSIX.
  • PALBSD_Aligned extends PALBSD to provide support for aligned allocation from mmap, as supported by NetBSD and FreeBSD.

Each of these template classes takes the PAL that inherits from it as a template parameter. A purely POSIX-compliant platform could have a PAL as simple as this:

class PALMyOS : public PALPOSIX<PALMyOS> {}

Typically, a PAL will implement at least one of the functions outlined above in a more-efficient platform-specific way, but this is not required. Non-POSIX systems will need to implement the entire PAL interface. The Windows, and OpenEnclave and FreeBSD kernel implementations give examples of non-POSIX environments that snmalloc supports.

The POSIX PAL uses mmap to map memory. Some POSIX or POSIX-like systems require minor tweaks to this behaviour. Rather than requiring these to copy and paste the code, a PAL that inherits from the POSIX PAL can define one or both of these (static constexpr) fields to customise the mmap behaviour.

  • default_mmap_flags allows a PAL to provide additional MAP_* flags to all mmap calls.
  • anonymous_memory_fd allows the PAL to override the default file descriptor used for memory mappings.