linux-kernel - Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable

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Message-ID: <75224ad1-f160-802a-9d72-b092ba864fb7@suse.com>
Date:   Fri, 13 Mar 2020 09:26:58 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>, boris.ostrovsky@...cle.com,
        sstabellini@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, jpoimboe@...hat.com
Cc:     x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        jslaby@...e.cz
Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks
 reliable

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@...e.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