Support (downstream) kernel-side unaligned atomic handling (The rebase sequel) #4985
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…ed atomic Default to enabled because this is the config we expect by default. In the future will get some benchmarking in various games like Assassin's Creed, and Call of Duty.
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This patch proposes adding kernel-side emulation for unaligned atomic instructions on ARM64. This is intended for x86 emulators (like FEX) that struggle to effectively handle such operations in userspace alone. Such handling is required as x86 permits such unaligned accesses (albeit sometimes with a performance penalty as in the case of split-locks[1]) but ARM64 does not and will raise a bus error. User applications that wish to enable support for this can use the new pctrl() flag `PR_ARM64_UNALIGN_ATOMIC_EMULATE`. Some optimizations and instructions were left for future revisions of this patchset. Emulators like FEX attempt to emulate this in userspace, but with caveats in two areas: * Performance It should first be noted that due to x86's TSO (total store order) memory model, ARM64 atomic instructions must be used for all memory accesses. This results in unaligned loads/stores being much more common than one would expect and the overhead of emulating them significantly impacting performance. For this common case of unaligned loads/stores, code backpatching is used in FEX to avoid repeated overhead from handling the same faulting access. This replaces faulting unaligned atomic ARM64 instructions with regular load/stores and memory barriers. This comes at a cost of introducing significant performance problems if a function like memcpy ends up being patched because it very infrequently happens to be used with unaligned memory. This is severe enough to make games like Mirror's Edge and Assassin's Creed: Origin unplayable without application-specific configuration. Microbenchmarks[2] measure a 4x decrease in overhead with kernel-side handling compared to userspace, and this figure is currently even larger when FEX is ran under Wine. Such a dramatic decrease would make it reasonable for FEX to default to the no-backpatching path and provide consistent performance. * Correctness: x86 atomic accesses can cross 16-byte (LSE2) granules, but there is no ARM64 instruction that would allow for direct atomic emulation of this. As such, a lock must be taken for correctness. While this is easy to emulate in userspace within a process, correct atomic cross-granule operations on shared memory mapped into multiple processes would require a lock shared between all FEX instances which cannot be implemented safely in userspace as is (robust futexes do not work here as they are under the control of the emulated x86 program). Note, this is a less coarse restriction than split locks on x86, which are only concerned with accesses crossing a 64 byte cacheline size. * Precedent: Both XNU and NT kernels support unaligned atomic emulation for their respective x86 emulators. Windows additionally supports 'volatile metadata', which is emitted by newer versions of MSVC to inform emulators which specific load/store accesses require atomic handling [3]. FEX supports this together with an extension mechanism [4] which can be manually populated to avoid e.g. the aforementioned Assassin's Creed slowdown. This implementation is a RFC so we can learn more about how to make this code upstream and what the maintainers think of such feature being merged here. The code is a simplified version of the original work done by Billy Laws, where we accept just a subset of 64bit atomic instructions that are enough to be used with a benchmark tool[2], and this is the proposed interface being used by FEX: [5]. Thanks! André [1] https://lwn.net/Articles/911219/ [2] https://gitlab.freedesktop.org/freedesktop/snippets/-/snippets/7875 [3] https://learn.microsoft.com/en-us/cpp/build/reference/volatile?view=msvc-170 [4] FEX-Emu/FEX#4773 [5] FEX-Emu/FEX#4985 To: Catalin Marinas <[email protected]> To: Will Deacon <[email protected]> Cc: Billy Laws <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: Mark Rutland <[email protected]> Cc: Mark Brown <[email protected]> Cc: Ryan Houdek <[email protected]> --- b4-submit-tracking --- # This section is used internally by b4 prep for tracking purposes. { "series": { "revision": 1, "change-id": "20251106-tonyk-unaligned_atomic-c6ca37947bfc", "prefixes": [ "RFC" ] } }
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Rebased version of #4765
Paranoid TSO is known broken, but it's already broken on main. Nothing of value was lost. I'll fix that later or whatever.