x86/fpu: Exclude dynamic states from init_fpstate

== Background ==

The XSTATE init code initializes all enabled and supported components.
Then, the init states are saved in the init_fpstate buffer that is
statically allocated in about one page.

The AMX TILE_DATA state is large (8KB) but its init state is zero. And the
feature comes only with the compacted format with these established
dependencies: AMX->XFD->XSAVES. So this state is excludable from
init_fpstate.

== Problem ==

But the buffer is formatted to include that large state. Then, this can be
the cause of a noisy splat like the below.

This came from XRSTORS for the task with init_fpstate in its XSAVE buffer.
It is reproducible on AMX systems when the running kernel is built with
CONFIG_DEBUG_PAGEALLOC=y and CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT=y:

 Bad FPU state detected at restore_fpregs_from_fpstate+0x57/0xd0, reinitializing FPU registers.
 ...
 RIP: 0010:restore_fpregs_from_fpstate+0x57/0xd0
  ? restore_fpregs_from_fpstate+0x45/0xd0
  switch_fpu_return+0x4e/0xe0
  exit_to_user_mode_prepare+0x17b/0x1b0
  syscall_exit_to_user_mode+0x29/0x40
  do_syscall_64+0x67/0x80
  ? do_syscall_64+0x67/0x80
  ? exc_page_fault+0x86/0x180
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

== Solution ==

Adjust init_fpstate to exclude dynamic states. XRSTORS from init_fpstate
still initializes those states when their bits are set in the
requested-feature bitmap.

Fixes: 2308ee5 ("x86/fpu/amx: Enable the AMX feature in 64-bit mode")
Reported-by: Lin X Wang <[email protected]>
Signed-off-by: Chang S. Bae <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Lin X Wang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ChangSeokBae

arch/x86/kernel/fpu/xstate.c

@@ -857,9 +857,12 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	update_regset_xstate_info(fpu_user_cfg.max_size,
 				  fpu_user_cfg.max_features);
 
-	/* Bring init_fpstate size and features up to date */
-	init_fpstate.size		= fpu_kernel_cfg.max_size;
-	init_fpstate.xfeatures		= fpu_kernel_cfg.max_features;
+	/*
+	 * init_fpstate excludes dynamic states as they are large but init
+	 * state is zero.
+	 */
+	init_fpstate.size		= fpu_kernel_cfg.default_size;
+	init_fpstate.xfeatures		= fpu_kernel_cfg.default_features;
 
 	if (init_fpstate.size > sizeof(init_fpstate.regs)) {
 		pr_warn("x86/fpu: init_fpstate buffer too small (%zu < %d), disabling XSAVE\n",