[PATCH 7/9] x86/fpu: Remove init_task FPU state dependencies, add debugging warning for PF_KTHREAD tasks

[Ingo Molnar <mingo@kernel.org>]

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 context
in any case, so remove the hack and add in some debug warnings.

As Linus noted in the discussion, the init task (and other
PF_KTHREAD tasks) *can* use the FPU via kernel_fpu_begin()/_end(),
but they don't need the context area because their FPU use is not
preemptible or reentrant, and they don't return to user-space.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20240605083557.2051480-4-mingo@kernel.org
---
 arch/x86/include/asm/processor.h |  6 +++++-
 arch/x86/kernel/fpu/core.c       | 15 +++++++++++----
 arch/x86/kernel/fpu/init.c       |  3 +--
 arch/x86/kernel/fpu/xstate.c     |  3 ---
 arch/x86/kernel/vmlinux.lds.S    |  4 ----
 5 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 3de609aad0af..4fd3364dbc73 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 52d5843c886c..db608afa686f 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -51,6 +51,16 @@ 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)
+{
+	if (WARN_ON_ONCE(task->flags & PF_KTHREAD))
+		return NULL;
+
+	return (void *)task + sizeof(*task);
+}
+#endif
+
 /*
  * Can we use the FPU in kernel mode with the
  * whole "kernel_fpu_begin/end()" sequence?
@@ -590,11 +600,9 @@ 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 *src_fpu = x86_task_fpu(current);
-	struct fpu *dst_fpu = x86_task_fpu(dst);
+	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
 
 	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 +665,6 @@ 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 794682b52373..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");
@@ -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