linux-kernel - Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in __preempt_enable

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Date:   Thu, 7 May 2020 16:15:41 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in
 __preempt_enable_notrace() thunk



On 5/5/20 3:41 PM, Thomas Gleixner wrote:
> The preempt_enable_notrace() ASM thunk is called from tracing, entry code
> RCU and other places which are already in or going to be in the noinstr
> section which protects sensitve code from being instrumented.

typo: "sensitve"

alex.

> Calls out of these sections happen with interrupts disabled, which is
> handled in C code, but the push regs, call, pop regs sequence can be
> completely avoided in this case.
> 
> This is also a preparatory step for annotating the call from the thunk to
> preempt_enable_notrace() safe from a noinstr section.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   arch/x86/entry/thunk_64.S       |   27 +++++++++++++++++++++++----
>   arch/x86/include/asm/irqflags.h |    3 +--
>   arch/x86/include/asm/paravirt.h |    3 +--
>   3 files changed, 25 insertions(+), 8 deletions(-)
> 
> --- a/arch/x86/entry/thunk_64.S
> +++ b/arch/x86/entry/thunk_64.S
> @@ -9,10 +9,28 @@
>   #include "calling.h"
>   #include <asm/asm.h>
>   #include <asm/export.h>
> +#include <asm/irqflags.h>
> +
> +.code64
>   
>   	/* rdi:	arg1 ... normal C conventions. rax is saved/restored. */
> -	.macro THUNK name, func, put_ret_addr_in_rdi=0
> +	.macro THUNK name, func, put_ret_addr_in_rdi=0, check_if=0
>   SYM_FUNC_START_NOALIGN(\name)
> +
> +	.if \check_if
> +	/*
> +	 * Check for interrupts disabled right here. No point in
> +	 * going all the way down
> +	 */
> +	pushq	%rax
> +	SAVE_FLAGS(CLBR_RAX)
> +	testl	$X86_EFLAGS_IF, %eax
> +	popq	%rax
> +	jnz	1f
> +	ret
> +1:
> +	.endif
> +
>   	pushq %rbp
>   	movq %rsp, %rbp
>   
> @@ -38,14 +56,15 @@ SYM_FUNC_END(\name)
>   	.endm
>   
>   #ifdef CONFIG_TRACE_IRQFLAGS
> -	THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller,1
> -	THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller,1
> +	THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller, put_ret_addr_in_rdi=1
> +	THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller, put_ret_addr_in_rdi=1
>   #endif
>   
>   #ifdef CONFIG_PREEMPTION
>   	THUNK preempt_schedule_thunk, preempt_schedule
> -	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
>   	EXPORT_SYMBOL(preempt_schedule_thunk)
> +
> +	THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace, check_if=1
>   	EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
>   #endif
>   
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -127,9 +127,8 @@ static inline notrace unsigned long arch
>   #define DISABLE_INTERRUPTS(x)	cli
>   
>   #ifdef CONFIG_X86_64
> -#ifdef CONFIG_DEBUG_ENTRY
> +
>   #define SAVE_FLAGS(x)		pushfq; popq %rax
> -#endif
>   
>   #define SWAPGS	swapgs
>   /*
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -907,14 +907,13 @@ extern void default_banner(void);
>   		  ANNOTATE_RETPOLINE_SAFE;				\
>   		  jmp PARA_INDIRECT(pv_ops+PV_CPU_usergs_sysret64);)
>   
> -#ifdef CONFIG_DEBUG_ENTRY
>   #define SAVE_FLAGS(clobbers)                                        \
>   	PARA_SITE(PARA_PATCH(PV_IRQ_save_fl),			    \
>   		  PV_SAVE_REGS(clobbers | CLBR_CALLEE_SAVE);        \
>   		  ANNOTATE_RETPOLINE_SAFE;			    \
>   		  call PARA_INDIRECT(pv_ops+PV_IRQ_save_fl);	    \
>   		  PV_RESTORE_REGS(clobbers | CLBR_CALLEE_SAVE);)
> -#endif
> +
>   #endif /* CONFIG_PARAVIRT_XXL */
>   #endif	/* CONFIG_X86_64 */
>   
>