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

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Message-ID: <CALCETrV79pw7-nisp4VdEkQ4=fr2nfJFOMCtyKmWZR6PG3=oWg@mail.gmail.com>
Date:   Thu, 24 Oct 2019 09:24:13 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.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 2:30 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> 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.

Well, I feel a bit silly.  I read this:

>
> >  #define SYSCALL_EXIT_WORK_FLAGS                                \
> > @@ -279,6 +282,9 @@ static void syscall_slow_exit_work(struc

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

and I applied the diff in my head to the wrong function, and I didn't
notice that it didn't really apply there.  Oddly, gitweb gets this
right:

https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=WIP.core/entry&id=e3158f93138ded84eb44fa97606197f6adcf9366

Looking at the actual code:

Acked-by: Andy Lutomirski <luto@...nel.org>

with one minor caveat: you are making a subtle and mostly irrelevant
semantic change: with your patch, user mode will be traced as IRQs on
even if a nasty user has used iopl() to turn off interrupts.  This is
probably a good thing, but I think you should mention it in the
changelog.

FWIW, the rest of the series looks pretty good, too.