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

["Ning, Hongyu" <hongyu.ning@linux.intel.com>]

On 2024/6/5 16:35, Ingo Molnar wrote:
> 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@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/ZgaNs1lC2Y+AnRG4@gmail.com
> ---
>   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

Hi,

we've hit x86/fpu related WARNING and NULL pointer issue during KVM/QEMU 
VM booting with latest linux-next kernel, bisect results show it's 
related to this commit, would you take a look?

detailed description in https://bugzilla.kernel.org/show_bug.cgi?id=218980