Re: [patch part-II V2 12/13] x86/entry: Move irq flags tracing to prepare_exit_to_usermode()

[Alexandre Chartre]

On 3/8/20 11:24 PM, Thomas Gleixner wrote:
> This is another step towards more C-code and less convoluted ASM.
>
> Note, that trace_hardirqs_on() is still incorrect vs. RCU idle when the
> tracepoint is used by e.g. BPF. Will be addressed in the next step.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>
> V2: New patch simplifying the conversion and addressing Alex' review
>      comment of redundant tracing.
> ---
>   arch/x86/entry/common.c          |    1 +
>   arch/x86/entry/entry_32.S        |   12 ++++--------
>   arch/x86/entry/entry_64.S        |    4 ----
>   arch/x86/entry/entry_64_compat.S |   14 +++++---------
>   4 files changed, 10 insertions(+), 21 deletions(-)


Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>

alex.

> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -253,6 +253,7 @@ static noinline void __prepare_exit_to_u
>   
>   	user_enter_irqoff();
>   	mds_user_clear_cpu_buffers();
> +	trace_hardirqs_on();
>   }
>   NOKPROBE_SYMBOL(prepare_exit_to_usermode);
>   
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -811,8 +811,7 @@ SYM_CODE_START(ret_from_fork)
>   	/* When we fork, we trace the syscall return in the child, too. */
>   	movl    %esp, %eax
>   	call    syscall_return_slowpath
> -	STACKLEAK_ERASE
> -	jmp     restore_all
> +	jmp     .Lsyscall_32_done
>   
>   	/* kernel thread */
>   1:	movl	%edi, %eax
> @@ -855,7 +854,7 @@ SYM_CODE_START_LOCAL(ret_from_exception)
>   	TRACE_IRQS_OFF
>   	movl	%esp, %eax
>   	call	prepare_exit_to_usermode
> -	jmp	restore_all
> +	jmp	restore_all_switch_stack
>   SYM_CODE_END(ret_from_exception)
>   
>   SYM_ENTRY(__begin_SYSENTER_singlestep_region, SYM_L_GLOBAL, SYM_A_NONE)
> @@ -968,8 +967,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
>   
>   	STACKLEAK_ERASE
>   
> -/* Opportunistic SYSEXIT */
> -	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
> +	/* Opportunistic SYSEXIT */
>   
>   	/*
>   	 * Setup entry stack - we keep the pointer in %eax and do the
> @@ -1072,11 +1070,9 @@ SYM_FUNC_START(entry_INT80_32)
>   	movl	%esp, %eax
>   	call	do_int80_syscall_32
>   .Lsyscall_32_done:
> -
>   	STACKLEAK_ERASE
>   
> -restore_all:
> -	TRACE_IRQS_ON
> +restore_all_switch_stack:
>   	SWITCH_TO_ENTRY_STACK
>   	CHECK_AND_APPLY_ESPFIX
>   
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -172,8 +172,6 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_h
>   	movq	%rsp, %rsi
>   	call	do_syscall_64		/* returns with IRQs disabled */
>   
> -	TRACE_IRQS_ON			/* return enables interrupts */
> -
>   	/*
>   	 * Try to use SYSRET instead of IRET if we're returning to
>   	 * a completely clean 64-bit userspace context.  If we're not,
> @@ -340,7 +338,6 @@ SYM_CODE_START(ret_from_fork)
>   	UNWIND_HINT_REGS
>   	movq	%rsp, %rdi
>   	call	syscall_return_slowpath	/* returns with IRQs disabled */
> -	TRACE_IRQS_ON			/* user mode is traced as IRQS on */
>   	jmp	swapgs_restore_regs_and_return_to_usermode
>   
>   1:
> @@ -617,7 +614,6 @@ SYM_CODE_START_LOCAL(common_interrupt)
>   .Lretint_user:
>   	mov	%rsp,%rdi
>   	call	prepare_exit_to_usermode
> -	TRACE_IRQS_ON
>   
>   SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
>   #ifdef CONFIG_DEBUG_ENTRY
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -132,8 +132,8 @@ SYM_FUNC_START(entry_SYSENTER_compat)
>   	movq	%rsp, %rdi
>   	call	do_fast_syscall_32
>   	/* XEN PV guests always use IRET path */
> -	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> -		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> +	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> +		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>   	jmp	sysret32_from_system_call
>   
>   .Lsysenter_fix_flags:
> @@ -244,8 +244,8 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
>   	movq	%rsp, %rdi
>   	call	do_fast_syscall_32
>   	/* XEN PV guests always use IRET path */
> -	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> -		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> +	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> +		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>   
>   	/* Opportunistic SYSRET */
>   sysret32_from_system_call:
> @@ -254,7 +254,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
>   	 * stack. So let's erase the thread stack right now.
>   	 */
>   	STACKLEAK_ERASE
> -	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
> +
>   	movq	RBX(%rsp), %rbx		/* pt_regs->rbx */
>   	movq	RBP(%rsp), %rbp		/* pt_regs->rbp */
>   	movq	EFLAGS(%rsp), %r11	/* pt_regs->flags (in r11) */
> @@ -393,9 +393,5 @@ SYM_CODE_START(entry_INT80_compat)
>   
>   	movq	%rsp, %rdi
>   	call	do_int80_syscall_32
> -.Lsyscall_32_done:
> -
> -	/* Go back to user mode. */
> -	TRACE_IRQS_ON
>   	jmp	swapgs_restore_regs_and_return_to_usermode
>   SYM_CODE_END(entry_INT80_compat)