linux-kernel - Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C for simple idtentries

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Message-ID: <20200226171705.GC6075@lenoir>
Date:   Wed, 26 Feb 2020 18:17:06 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C
 for simple idtentries

On Wed, Feb 26, 2020 at 09:09:34AM -0800, Andy Lutomirski wrote:
> 
> 
> > On Feb 26, 2020, at 9:05 AM, Frederic Weisbecker <frederic@...nel.org> wrote:
> > 
> > On Wed, Feb 26, 2020 at 09:05:38AM +0100, Peter Zijlstra wrote:
> >>> On Tue, Feb 25, 2020 at 11:33:34PM +0100, Thomas Gleixner wrote:
> >>> 
> >>> --- a/arch/x86/include/asm/idtentry.h
> >>> +++ b/arch/x86/include/asm/idtentry.h
> >>> @@ -7,14 +7,31 @@
> >>> 
> >>> #ifndef __ASSEMBLY__
> >>> 
> >>> +#ifdef CONFIG_CONTEXT_TRACKING
> >>> +static __always_inline void enter_from_user_context(void)
> >>> +{
> >>> +    CT_WARN_ON(ct_state() != CONTEXT_USER);
> >>> +    user_exit_irqoff();
> >>> +}
> >>> +#else
> >>> +static __always_inline void enter_from_user_context(void) { }
> >>> +#endif
> >>> +
> >>> /**
> >>>  * idtentry_enter - Handle state tracking on idtentry
> >>>  * @regs:    Pointer to pt_regs of interrupted context
> >>>  *
> >>> - * Place holder for now.
> >>> + * Invokes:
> >>> + *  - The hardirq tracer to keep the state consistent as low level ASM
> >>> + *    entry disabled interrupts.
> >>> + *
> >>> + *  - Context tracking if the exception hit user mode
> >>>  */
> >>> static __always_inline void idtentry_enter(struct pt_regs *regs)
> >>> {
> >>> +    trace_hardirqs_off();
> >>> +    if (user_mode(regs))
> >>> +        enter_from_user_context();
> >>> }
> >> 
> >> So:
> >> 
> >>    asm_exc_int3
> >>      exc_int3
> >>        idtentry_enter()
> >>          enter_from_user_context
> >>            if (context_tracking_enabled())
> >> 
> >>        poke_int3_handler();
> >> 
> >> Is, AFAICT, completely buggered.
> >> 
> >> You can't have a static_branch before the poke_int3_handler that deals
> >> with text_poke.
> > 
> > #BP is treated like an NMI in your patchset IIRC?
> > In that case and since that implies we can't schedule, we can remove
> > the call to context tracking there once we have nmi_enter()/nmi_exit()
> > called unconditionally.
> 
> int3 from user mode can send signals. This has better be able to schedule by the time it hits prepare_exit_to_usermode.

Oh right...