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

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Message-ID: <dc55b23b-c0d2-3be0-222f-d104548c8cf4@citrix.com>
Date:   Thu, 12 Mar 2020 15:04:36 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Miroslav Benes <mbenes@...e.cz>, <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>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <live-patching@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        <jslaby@...e.cz>
Subject: Re: [Xen-devel] [PATCH 1/2] x86/xen: Make the boot CPU idle task
 reliable

On 12/03/2020 14:20, 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.
>
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> ---
>  arch/x86/xen/xen-head.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
> index 1d0cee3163e4..642f346bfe02 100644
> --- a/arch/x86/xen/xen-head.S
> +++ b/arch/x86/xen/xen-head.S
> @@ -35,7 +35,7 @@ SYM_CODE_START(startup_xen)
>  	rep __ASM_SIZE(stos)
>  
>  	mov %_ASM_SI, xen_start_info
> -	mov $init_thread_union+THREAD_SIZE, %_ASM_SP
> +	mov $init_thread_union+THREAD_SIZE-SIZEOF_PTREGS, %_ASM_SP
>  
>  #ifdef CONFIG_X86_64
>  	/* Set up %gs.
> @@ -51,7 +51,9 @@ SYM_CODE_START(startup_xen)
>  	wrmsr
>  #endif
>  
> +	push $1f
>  	jmp xen_start_kernel
> +1:

Hang on.  Isn't this just a `call` instruction written in longhand?

~Andrew