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

Commit aefbab8e77

  ("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>

arch/arm64/include/asm/fpu.h

@@ -15,12 +15,12 @@ static inline void kernel_fpu_begin(void)
 {
 	BUG_ON(!in_task());
 	preempt_disable();
-	kernel_neon_begin();
+	kernel_neon_begin(NULL);
 }
 
 static inline void kernel_fpu_end(void)
 {
-	kernel_neon_end();
+	kernel_neon_end(NULL);
 	preempt_enable();
 }
 

arch/arm64/include/asm/neon.h

@@ -13,7 +13,7 @@
 
 #define cpu_has_neon()		system_supports_fpsimd()
 
-void kernel_neon_begin(void);
-void kernel_neon_end(void);
+void kernel_neon_begin(struct user_fpsimd_state *);
+void kernel_neon_end(struct user_fpsimd_state *);
 
 #endif /* ! __ASM_NEON_H */

arch/arm64/include/asm/processor.h

@@ -172,7 +172,12 @@ struct thread_struct {
 	unsigned long		fault_code;	/* ESR_EL1 value */
 	struct debug_info	debug;		/* debugging */
 
-	struct user_fpsimd_state	kernel_fpsimd_state;
+	/*
+	 * Set [cleared] by kernel_neon_begin() [kernel_neon_end()] to the
+	 * address of a caller provided buffer that will be used to preserve a
+	 * task's kernel mode FPSIMD state while it is scheduled out.
+	 */
+	struct user_fpsimd_state	*kernel_fpsimd_state;
 	unsigned int			kernel_fpsimd_cpu;
 #ifdef CONFIG_ARM64_PTR_AUTH
 	struct ptrauth_keys_user	keys_user;

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

arch/arm64/kernel/fpsimd.c

@@ -1489,21 +1489,23 @@ static void fpsimd_load_kernel_state(struct task_struct *task)
 	 * Elide the load if this CPU holds the most recent kernel mode
 	 * FPSIMD context of the current task.
 	 */
-	if (last->st == &task->thread.kernel_fpsimd_state &&
+	if (last->st == task->thread.kernel_fpsimd_state &&
 	    task->thread.kernel_fpsimd_cpu == smp_processor_id())
 		return;
 
-	fpsimd_load_state(&task->thread.kernel_fpsimd_state);
+	fpsimd_load_state(task->thread.kernel_fpsimd_state);
 }
 
 static void fpsimd_save_kernel_state(struct task_struct *task)
 {
 	struct cpu_fp_state cpu_fp_state = {
-		.st		= &task->thread.kernel_fpsimd_state,
+		.st		= task->thread.kernel_fpsimd_state,
 		.to_save	= FP_STATE_FPSIMD,
 	};
 
-	fpsimd_save_state(&task->thread.kernel_fpsimd_state);
+	BUG_ON(!cpu_fp_state.st);
+
+	fpsimd_save_state(task->thread.kernel_fpsimd_state);
 	fpsimd_bind_state_to_cpu(&cpu_fp_state);
 
 	task->thread.kernel_fpsimd_cpu = smp_processor_id();
@@ -1774,6 +1776,7 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
 void fpsimd_flush_task_state(struct task_struct *t)
 {
 	t->thread.fpsimd_cpu = NR_CPUS;
+	t->thread.kernel_fpsimd_state = NULL;
 	/*
 	 * If we don't support fpsimd, bail out after we have
 	 * reset the fpsimd_cpu for this task and clear the
@@ -1833,12 +1836,19 @@ void fpsimd_save_and_flush_cpu_state(void)
  *
  * The caller may freely use the FPSIMD registers until kernel_neon_end() is
  * called.
+ *
+ * Unless called from non-preemptible task context, @state must point to a
+ * caller provided buffer that will be used to preserve the task's kernel mode
+ * FPSIMD context when it is scheduled out, or if it is interrupted by kernel
+ * mode FPSIMD occurring in softirq context. May be %NULL otherwise.
  */
-void kernel_neon_begin(void)
+void kernel_neon_begin(struct user_fpsimd_state *state)
 {
 	if (WARN_ON(!system_supports_fpsimd()))
 		return;
 
+	WARN_ON((preemptible() || in_serving_softirq()) && !state);
+
 	BUG_ON(!may_use_simd());
 
 	get_cpu_fpsimd_context();
@@ -1846,7 +1856,7 @@ void kernel_neon_begin(void)
 	/* Save unsaved fpsimd state, if any: */
 	if (test_thread_flag(TIF_KERNEL_FPSTATE)) {
 		BUG_ON(IS_ENABLED(CONFIG_PREEMPT_RT) || !in_serving_softirq());
-		fpsimd_save_kernel_state(current);
+		fpsimd_save_state(state);
 	} else {
 		fpsimd_save_user_state();
 
@@ -1867,8 +1877,16 @@ void kernel_neon_begin(void)
 		 * mode in task context. So in this case, setting the flag here
 		 * is always appropriate.
 		 */
-		if (IS_ENABLED(CONFIG_PREEMPT_RT) || !in_serving_softirq())
+		if (IS_ENABLED(CONFIG_PREEMPT_RT) || !in_serving_softirq()) {
+			/*
+			 * Record the caller provided buffer as the kernel mode
+			 * FP/SIMD buffer for this task, so that the state can
+			 * be preserved and restored on a context switch.
+			 */
+			WARN_ON(current->thread.kernel_fpsimd_state != NULL);
+			current->thread.kernel_fpsimd_state = state;
 			set_thread_flag(TIF_KERNEL_FPSTATE);
+		}
 	}
 
 	/* Invalidate any task state remaining in the fpsimd regs: */
@@ -1886,22 +1904,30 @@ EXPORT_SYMBOL_GPL(kernel_neon_begin);
  *
  * The caller must not use the FPSIMD registers after this function is called,
  * unless kernel_neon_begin() is called again in the meantime.
+ *
+ * The value of @state must match the value passed to the preceding call to
+ * kernel_neon_begin().
  */
-void kernel_neon_end(void)
+void kernel_neon_end(struct user_fpsimd_state *state)
 {
 	if (!system_supports_fpsimd())
 		return;
 
+	if (!test_thread_flag(TIF_KERNEL_FPSTATE))
+		return;
+
 	/*
 	 * If we are returning from a nested use of kernel mode FPSIMD, restore
 	 * the task context kernel mode FPSIMD state. This can only happen when
 	 * running in softirq context on non-PREEMPT_RT.
 	 */
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && in_serving_softirq() &&
-	    test_thread_flag(TIF_KERNEL_FPSTATE))
-		fpsimd_load_kernel_state(current);
-	else
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && in_serving_softirq()) {
+		fpsimd_load_state(state);
+	} else {
 		clear_thread_flag(TIF_KERNEL_FPSTATE);
+		WARN_ON(current->thread.kernel_fpsimd_state != state);
+		current->thread.kernel_fpsimd_state = NULL;
+	}
 }
 EXPORT_SYMBOL_GPL(kernel_neon_end);
 
@@ -1937,7 +1963,7 @@ void __efi_fpsimd_begin(void)
 	WARN_ON(preemptible());
 
 	if (may_use_simd()) {
-		kernel_neon_begin();
+		kernel_neon_begin(&efi_fpsimd_state);
 	} else {
 		/*
 		 * If !efi_sve_state, SVE can't be in use yet and doesn't need
@@ -1986,7 +2012,7 @@ void __efi_fpsimd_end(void)
 		return;
 
 	if (!efi_fpsimd_state_used) {
-		kernel_neon_end();
+		kernel_neon_end(&efi_fpsimd_state);
 	} else {
 		if (system_supports_sve() && efi_sve_state_used) {
 			bool ffr = true;