linux-kernel - Re: [RFC][PATCH 3/3] x86/entry, ORC: Teach objtool/unwind

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Date:   Fri, 8 May 2020 14:40:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org
Subject: Re: [RFC][PATCH 3/3] x86/entry, ORC: Teach objtool/unwind_orc about
 stack irq swizzles

On Fri, May 08, 2020 at 02:26:32PM +0200, Thomas Gleixner wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Thu, May 07, 2020 at 11:24:49PM +0200, Thomas Gleixner wrote:
> >> But over our IRC conversation I came up with a 3rd variant:
> >> 
> >>   For most of the vectors the indirect call overhead is just noise, so
> >>   we can run them through the ASM switcher, but for the resched IPI
> >>   we can just use a separate direct call stub in ASM.
> >
> > Are we sure the rat-poison crap is noise for all the other system
> > vectors? I suppose it is for most since they'll do indirect calls
> > themselves anyway, right?
> 
> We have different categories:
> 
>  1) Uninteresting
> 
>     SPURIOUS_APIC_VECTOR, ERROR_APIC_VECTOR, THERMAL_APIC_VECTOR,
>     THRESHOLD_APIC_VECTOR, REBOOT_VECTOR, DEFERRED_ERROR_VECTOR
> 
>  2) Indirect call poisoned
> 
>     LOCAL_TIMER_VECTOR
>     X86_PLATFORM_IPI_VECTOR
>     IRQ_WORK_VECTOR
>     HYPERV_STIMER0_VECTOR
>     HYPERVISOR_CALLBACK_VECTOR
>     POSTED_INTERRUPT_WAKEUP_VECTOR.
>     CALL_FUNCTION_VECTOR
>     CALL_FUNCTION_SINGLE_VECTOR
>     
>  3) Quick
> 
>     RESCHEDULE_VECTOR
> 
>     POSTED_INTR_VECTOR
>     POSTED_INTR_NESTED_VECTOR
> 
>         These two postit ones are weird because they are both empty and
>         just increment different irq counts.
> 
>     HYPERV_REENLIGHTENMENT_VECTOR
> 
>         schedules delayed work, i,e. arms a timer which should be
>         straight forward, but does it matter?
> 
>  4) Others
> 
>     UV_BAU_MESSAGE - The TLB flushes are probably more expensive than
>                      ratpoutine
> 
> Hmm?

As we just agreed on IRC, 3) can run without changing stack, and then
the rest can use the indirect thing.