On Thu, 12 Mar 2020, Andrew Cooper wrote:

> 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@suse.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?

It is (as far as I know). I wanted to keep it opencoded for a reason I 
don't remember now. I'll change it. Thanks.

Miroslav