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Commit aefbab8e77eb16b5
("arm64: fpsimd: Preserve/restore kernel mode NEON at context switch")
added a 'kernel_fpsimd_state' field to struct thread_struct, which is
the arch-specific portion of struct task_struct, and is allocated for
each task in the system. The size of this field is 528 bytes, resulting
in non-negligible bloat of task_struct, and the resulting memory
overhead may impact performance on systems with many processes.
This allocation is only used if the task is scheduled out or interrupted
by a softirq while using the FP/SIMD unit in kernel mode, and so it is
possible to transparently allocate this buffer on the caller's stack
instead.
So tweak the 'ksimd' scoped guard implementation so that a stack buffer
is allocated and passed to both kernel_neon_begin() and
kernel_neon_end(), and either record it in the task struct, or use it
directly to preserve the task mode kernel FP/SIMD when running in
softirq context. Passing the address to both functions, and checking the
addresses for consistency ensures that callers of the updated bare
begin/end API use it in a manner that is consistent with the new context
switch semantics.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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The generic kernel mode FPU API, which is used by the AMDGPU driver to
perform floating point calculations, is modeled after the most
restrictive architecture that supports it. This means it doesn't support
preemption, and can only be used from task context.
The arm64 implementation is a bit more flexible, but supporting that in
the generic API complicates matters slightly, and for no good reason,
given that the only user does not need it.
So enforce that kernel_fpu_begin() can only be called from task context,
and [redundantly] disable preemption. This removes the need for users of
this API to provide a kernel mode FP/SIMD state after a future patch
that makes that compulsory for preemptible task context.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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arm64 provides an equivalent to the common kernel-mode FPU API, but in a
different header and using different function names. Add a wrapper
header, and export CFLAGS adjustments as found in lib/raid6/Makefile.
Link: https://lkml.kernel.org/r/20240329072441.591471-5-samuel.holland@sifive.com
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Palmer Dabbelt <palmer@rivosinc.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: WANG Xuerui <git@xen0n.name>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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