[PATCH 3/3] x86/fpu: Remove init_task FPU state dependencies, add debugging warning

[Ingo Molnar]

init_task's FPU state initialization was a bit of a hack:

		__x86_init_fpu_begin = .;
		. = __x86_init_fpu_begin + 128*PAGE_SIZE;
		__x86_init_fpu_end = .;

But the init task isn't supposed to be using the FPU in any case,
so remove the hack and add in some debug warnings.

Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
Link: https://lore.kernel.org/r/ZgaNs1lC2Y+AnRG4@xxxxxxxxx
---
 arch/x86/include/asm/processor.h |  6 +++++-
 arch/x86/kernel/fpu/core.c       | 12 +++++++++---
 arch/x86/kernel/fpu/init.c       |  5 ++---
 arch/x86/kernel/fpu/xstate.c     |  3 ---
 arch/x86/kernel/vmlinux.lds.S    |  4 ----
 5 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 249c5fa20de4..ed8981866f4d 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -504,7 +504,11 @@ struct thread_struct {
 #endif
 };
 
-#define x86_task_fpu(task) ((struct fpu *)((void *)task + sizeof(*task)))
+#ifdef CONFIG_X86_DEBUG_FPU
+extern struct fpu *x86_task_fpu(struct task_struct *task);
+#else
+# define x86_task_fpu(task) ((struct fpu *)((void *)task + sizeof(*task)))
+#endif
 
 /*
  * X86 doesn't need any embedded-FPU-struct quirks:
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 0ccabcd3bf62..fdc3b227800d 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -51,6 +51,15 @@ static DEFINE_PER_CPU(bool, in_kernel_fpu);
  */
 DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
+#ifdef CONFIG_X86_DEBUG_FPU
+struct fpu *x86_task_fpu(struct task_struct *task)
+{
+	WARN_ON_ONCE(task == &init_task);
+
+	return (void *)task + sizeof(*task);
+}
+#endif
+
 /*
  * Can we use the FPU in kernel mode with the
  * whole "kernel_fpu_begin/end()" sequence?
@@ -591,10 +600,8 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
 	 * This is safe because task_struct size is a multiple of cacheline size.
 	 */
 	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
-	struct fpu *src_fpu = x86_task_fpu(current);
 
 	BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);
-	BUG_ON(!src_fpu);
 
 	/* The new task's FPU state cannot be valid in the hardware. */
 	dst_fpu->last_cpu = -1;
@@ -657,7 +664,6 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
 	if (update_fpu_shstk(dst, ssp))
 		return 1;
 
-	trace_x86_fpu_copy_src(src_fpu);
 	trace_x86_fpu_copy_dst(dst_fpu);
 
 	return 0;
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 11aa31410df2..53580e59e5db 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -38,7 +38,7 @@ static void fpu__init_cpu_generic(void)
 	/* Flush out any pending x87 state: */
 #ifdef CONFIG_MATH_EMULATION
 	if (!boot_cpu_has(X86_FEATURE_FPU))
-		fpstate_init_soft(&x86_task_fpu(current)->fpstate->regs.soft);
+		;
 	else
 #endif
 		asm volatile ("fninit");
@@ -164,7 +164,7 @@ static void __init fpu__init_task_struct_size(void)
 	 * Subtract off the static size of the register state.
 	 * It potentially has a bunch of padding.
 	 */
-	task_size -= sizeof(x86_task_fpu(current)->__fpstate.regs);
+	task_size -= sizeof(union fpregs_state);
 
 	/*
 	 * Add back the dynamically-calculated register state
@@ -209,7 +209,6 @@ static void __init fpu__init_system_xstate_size_legacy(void)
 	fpu_kernel_cfg.default_size = size;
 	fpu_user_cfg.max_size = size;
 	fpu_user_cfg.default_size = size;
-	fpstate_reset(x86_task_fpu(current));
 }
 
 /*
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 90b11671e943..1f37da22ddbe 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -844,9 +844,6 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	if (err)
 		goto out_disable;
 
-	/* Reset the state for the current task */
-	fpstate_reset(x86_task_fpu(current));
-
 	/*
 	 * Update info used for ptrace frames; use standard-format size and no
 	 * supervisor xstates:
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 226244a894da..3509afc6a672 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -170,10 +170,6 @@ SECTIONS
 		/* equivalent to task_pt_regs(&init_task) */
 		__top_init_kernel_stack = __end_init_stack - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE;
 
-		__x86_init_fpu_begin = .;
-		. = __x86_init_fpu_begin + 128*PAGE_SIZE;
-		__x86_init_fpu_end = .;
-
 #ifdef CONFIG_X86_32
 		/* 32 bit has nosave before _edata */
 		NOSAVE_DATA
-- 
2.43.0