linux-kernel - Re: [RFC patch 10/15] x86/entry: Move irq tracing to C code

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Message-ID: <20190923115551.GZ2386@hirez.programming.kicks-ass.net>
Date:   Mon, 23 Sep 2019 13:55:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-arch@...r.kernel.org, jgross@...e.com
Subject: Re: [RFC patch 10/15] x86/entry: Move irq tracing to C code

On Mon, Sep 23, 2019 at 01:49:20PM +0200, Peter Zijlstra wrote:
> While walking the kids to school I wondered WTH we need to call
> TRACE_IRQS_OFF in the first place. If this is the return from exception
> path, interrupts had better be disabled already (in exception enter).
> 
> For entry_64.S we have:
> 
>   - idtentry_part; which does TRACE_IRQS_OFF at the start and error_exit
>     at the end.
> 
>   - xen_do_hypervisor_callback, xen_failsafe_callback -- which are
>     confusing.
> 
> So in the normal case, it appears we can simply do:
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index b7c3ea4cb19d..e9cf59ac554e 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1368,8 +1368,6 @@ END(error_entry)
>  
>  ENTRY(error_exit)
>  	UNWIND_HINT_REGS
> -	DISABLE_INTERRUPTS(CLBR_ANY)
> -	TRACE_IRQS_OFF
>  	testb	$3, CS(%rsp)
>  	jz	retint_kernel
>  	jmp	retint_user
> 
> and all should be well. This leaves Xen...
> 
> For entry_32.S it looks like nothing uses 'resume_userspace' so that
> ENTRY can go away. Which then leaves:
> 
>  * ret_from_intr:
>   - common_spurious: TRACE_IRQS_OFF
>   - common_interrupt: TRACE_IRQS_OFF
>   - BUILD_INTERRUPT3: TRACE_IRQS_OFF
>   - xen_do_upcall: ...
> 
>  * ret_from_exception:
>   - xen_failsafe_callback: ...
>   - common_exception_read_cr2: TRACE_IRQS_OFF
>   - common_exception: TRACE_IRQS_OFF
>   - int3: TRACE_IRQS_OFF
> 
> Which again suggests:
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index f83ca5aa8b77..7a19e7413a8e 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -825,9 +825,6 @@ END(ret_from_fork)
>  	cmpl	$USER_RPL, %eax
>  	jb	restore_all_kernel		# not returning to v8086 or userspace
>  
> -ENTRY(resume_userspace)
> -	DISABLE_INTERRUPTS(CLBR_ANY)
> -	TRACE_IRQS_OFF
>  	movl	%esp, %eax
>  	call	prepare_exit_to_usermode
>  	jmp	restore_all
> 
> with the notable exception (oh teh pun!) being Xen... _again_.
> 
> With these patchlets on, we'd want prepare_exit_to_usermode() to
> validate the IRQ state, but AFAICT it _should_ all just 'work' (famous
> last words).
> 
> Cc Juergen because Xen

Arrgh.. faults!! they do local_irq_enable() but never disable them
again. Arguably those functions should be symmetric and restore IF when
they muck with it instead of relying on the exit path fixing things up.