linux-kernel - Re: [patch V2 08/17] x86/entry: Move syscall irq tracing to C code

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Date:   Wed, 23 Oct 2019 14:30:51 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [patch V2 08/17] x86/entry: Move syscall irq tracing to C code

On Wed, Oct 23, 2019 at 5:31 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Interrupt state tracing can be safely done in C code. The few stack
> operations in assembly do not need to be covered.
>
> Remove the now pointless indirection via .Lsyscall_32_done and jump to
> swapgs_restore_regs_and_return_to_usermode directly.

This doesn't look right.

>  #define SYSCALL_EXIT_WORK_FLAGS                                \
> @@ -279,6 +282,9 @@ static void syscall_slow_exit_work(struc
>  {
>         struct thread_info *ti;
>
> +       /* User to kernel transition disabled interrupts. */
> +       trace_hardirqs_off();
> +

So you just traced IRQs off, but...

>         enter_from_user_mode();
>         local_irq_enable();

Now they're on and traced on again?

I also don't see how your patch handles the fastpath case.

How about the attached patch instead?

View attachment "irqtrace.patch" of type "text/x-patch" (2651 bytes)