Revert "arm64: fpsimd: Implement lazy restore for kernel mode FPSIMD"

This reverts commit 2632e25.

Johannes (and others) report data corruption with dm-crypt on Apple M1
which has been bisected to this change. Revert the offending commit
while we figure out what's going on.

Cc: [email protected]
Reported-by: Johannes Nixdorf <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Will Deacon <[email protected]>

Author: willdeacon

arch/arm64/include/asm/processor.h

@@ -171,7 +171,6 @@ struct thread_struct {
 	struct debug_info	debug;		/* debugging */
 
 	struct user_fpsimd_state	kernel_fpsimd_state;
-	unsigned int			kernel_fpsimd_cpu;
 #ifdef CONFIG_ARM64_PTR_AUTH
 	struct ptrauth_keys_user	keys_user;
 #ifdef CONFIG_ARM64_PTR_AUTH_KERNEL

arch/arm64/kernel/fpsimd.c

@@ -1509,30 +1509,12 @@ void do_fpsimd_exc(unsigned long esr, struct pt_regs *regs)
 
 static void fpsimd_load_kernel_state(struct task_struct *task)
 {
-	struct cpu_fp_state *last = this_cpu_ptr(&fpsimd_last_state);
-
-	/*
-	 * 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 &&
-	    task->thread.kernel_fpsimd_cpu == smp_processor_id())
-		return;
-
 	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,
-		.to_save	= FP_STATE_FPSIMD,
-	};
-
 	fpsimd_save_state(&task->thread.kernel_fpsimd_state);
-	fpsimd_bind_state_to_cpu(&cpu_fp_state);
-
-	task->thread.kernel_fpsimd_cpu = smp_processor_id();
 }
 
 void fpsimd_thread_switch(struct task_struct *next)