arm64/fpsimd: Allocate kernel mode FP/SIMD buffers on the stack

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>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index d9f83c478736..7ddb25df5c98 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -43,8 +43,11 @@ static __must_check inline bool may_use_simd(void) {
 
 #endif /* ! CONFIG_KERNEL_MODE_NEON */
 
-DEFINE_LOCK_GUARD_0(ksimd, kernel_neon_begin(), kernel_neon_end())
+DEFINE_LOCK_GUARD_1(ksimd,
+		    struct user_fpsimd_state,
+		    kernel_neon_begin(_T->lock),
+		    kernel_neon_end(_T->lock))
 
-#define scoped_ksimd()	scoped_guard(ksimd)
+#define scoped_ksimd()	scoped_guard(ksimd, &(struct user_fpsimd_state){})
 
 #endif