linux-kernel - Re: [PATCH v2 1/2] x86/xen: Make the boot CPU idle task reliable

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Message-ID: <71c4eeaf-958a-b215-3033-c3e0d74a9cfa@suse.com>
Date:   Thu, 19 Mar 2020 11:01:21 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, jpoimboe@...hat.com,
        andrew.cooper3@...rix.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        xen-devel@...ts.xenproject.org, jslaby@...e.cz
Subject: Re: [PATCH v2 1/2] x86/xen: Make the boot CPU idle task reliable

On 19.03.2020 10:56, Miroslav Benes wrote:
> The unwinder reports the boot CPU idle task's stack on XEN PV as
> unreliable, which affects at least live patching. There are two reasons
> for this. First, the task does not follow the x86 convention that its
> stack starts at the offset right below saved pt_regs. It allows the
> unwinder to easily detect the end of the stack and verify it. Second,
> startup_xen() function does not store the return address before jumping
> to xen_start_kernel() which confuses the unwinder.
> 
> Amend both issues by moving the starting point of initial stack in
> startup_xen() and storing the return address before the jump, which is
> exactly what call instruction does.
> 
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> ---
>  arch/x86/xen/xen-head.S | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 1d0cee3163e4..edc776af0e0a 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -35,7 +35,11 @@ SYM_CODE_START(startup_xen)
>  	rep __ASM_SIZE(stos)
>  
>  	mov %_ASM_SI, xen_start_info
> -	mov $init_thread_union+THREAD_SIZE, %_ASM_SP
> +#ifdef CONFIG_X86_64
> +	mov initial_stack(%rip), %_ASM_SP
> +#else
> +	mov pa(initial_stack), %_ASM_SP
> +#endif

If you need to distinguish the two anyway, why not use %rsp and
%esp respectively?

Jan