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Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks
 reliable
To:     Miroslav Benes <mbenes@suse.cz>, boris.ostrovsky@oracle.com,
        sstabellini@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, jpoimboe@redhat.com
Cc:     x86@kernel.org, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, live-patching@vger.kernel.org,
        jslaby@suse.cz
References: <20200312142007.11488-1-mbenes@suse.cz>
 <20200312142007.11488-3-mbenes@suse.cz>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
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Date:   Fri, 13 Mar 2020 09:26:58 +0100
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On 12.03.20 15:20, Miroslav Benes wrote:
> The unwinder reports the secondary CPU idle tasks' stack on XEN PV as
> unreliable, which affects at least live patching.
> cpu_initialize_context() sets up the context of the CPU through
> VCPUOP_initialise hypercall. After it is woken up, the idle task starts
> in cpu_bringup_and_idle() function and its stack starts at the offset
> right below pt_regs. The unwinder correctly detects the end of stack
> there but it is confused by NULL return address in the last frame.
> 
> RFC: I haven't found the way to teach the unwinder about the state of
> the stack there. Thus the ugly hack using assembly. Similar to what
> startup_xen() has got for boot CPU.
> 
> It introduces objtool "unreachable instruction" warning just right after
> the jump to cpu_bringup_and_idle(). It should show the idea what needs
> to be done though, I think. Ideas welcome.
> 
> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> ---
>   arch/x86/xen/smp_pv.c   |  3 ++-
>   arch/x86/xen/xen-head.S | 10 ++++++++++
>   2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> index 802ee5bba66c..6b88cdcbef8f 100644
> --- a/arch/x86/xen/smp_pv.c
> +++ b/arch/x86/xen/smp_pv.c
> @@ -53,6 +53,7 @@ static DEFINE_PER_CPU(struct xen_common_irq, xen_irq_work) = { .irq = -1 };
>   static DEFINE_PER_CPU(struct xen_common_irq, xen_pmu_irq) = { .irq = -1 };
>   
>   static irqreturn_t xen_irq_work_interrupt(int irq, void *dev_id);
> +extern unsigned char asm_cpu_bringup_and_idle[];
>   
>   static void cpu_bringup(void)
>   {

Would adding this here work?

+	asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));


Juergen