[PATCH 0/2] x86/xen: Make idle tasks reliable

[PATCH 0/2] x86/xen: Make idle tasks reliable

[Miroslav Benes]

The unwinder reports idle tasks' stack on XEN PV as unreliable which
complicates things for at least live patching. The two patches in the
series try to amend that by using similar approach as non-XEN x86 does.

However, I did not come up with a nice solution for secondary CPUs idle
tasks. The patch just shows the idea what should be done but it is an
ugly hack. Ideas are more than welcome.

Miroslav Benes (2):
  x86/xen: Make the boot CPU idle task reliable
  x86/xen: Make the secondary CPU idle tasks reliable

 arch/x86/xen/smp_pv.c   |  3 ++-
 arch/x86/xen/xen-head.S | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

-- 
2.25.1

[PATCH 1/2] x86/xen: Make the boot CPU idle task reliable

[Miroslav Benes]

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:
 SYM_CODE_END(startup_xen)
 	__FINIT
 #endif
-- 
2.25.1

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

[Miroslav Benes]

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)
 {
@@ -309,7 +310,7 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
 	 * pointing just below where pt_regs would be if it were a normal
 	 * kernel entry.
 	 */
-	ctxt->user_regs.eip = (unsigned long)cpu_bringup_and_idle;
+	ctxt->user_regs.eip = (unsigned long)asm_cpu_bringup_and_idle;
 	ctxt->flags = VGCF_IN_KERNEL;
 	ctxt->user_regs.eflags = 0x1000; /* IOPL_RING1 */
 	ctxt->user_regs.ds = __USER_DS;
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 642f346bfe02..c9a9c0bb79ed 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -56,6 +56,16 @@ SYM_CODE_START(startup_xen)
 1:
 SYM_CODE_END(startup_xen)
 	__FINIT
+
+.pushsection .text
+SYM_CODE_START(asm_cpu_bringup_and_idle)
+	UNWIND_HINT_EMPTY
+
+	push $1f
+	jmp cpu_bringup_and_idle
+1:
+SYM_CODE_END(asm_cpu_bringup_and_idle)
+.popsection
 #endif
 
 .pushsection .text
-- 
2.25.1

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

[Andrew Cooper]

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?

~Andrew

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

[Miroslav Benes]

[-- Attachment #1: Type: text/plain, Size: 1678 bytes --]

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

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

[Jürgen Groß]

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

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

[Miroslav Benes]

[-- Attachment #1: Type: text/plain, Size: 2944 bytes --]

On Fri, 13 Mar 2020, Jürgen Groß wrote:

> 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));

I tried something similar. It did not work, because than the hint is 
"bound" to the closest next call in the function which is cr4_init() in 
this case. The unwinder would not take it into account.

In my case, I placed it at the beginning of cpu_bringup_and_idle(). I also 
open coded it and played with the offset in the orc entry, but that did 
not work for some other reason.

However, now I tried this

diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 6b88cdcbef8f..39afd88309cb 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
 {
        cpu_bringup();
        boot_init_stack_canary();
+       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
        cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }

and that seems to work. I need to properly verify and test, but the 
explanation is that as opposed to the above, cpu_startup_entry() is on the 
idle task's stack and the hint is then taken into account. The unwound 
stack seems to be complete, so it could indeed be the fix.

Thanks
Miroslav

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

[Boris Ostrovsky]

On 3/12/20 10:20 AM, Miroslav Benes wrote:
> --- 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

This is initial_stack, isn't it?

-boris

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

[Miroslav Benes]

[-- Attachment #1: Type: text/plain, Size: 3655 bytes --]

On Fri, 13 Mar 2020, Miroslav Benes wrote:

> On Fri, 13 Mar 2020, Jürgen Groß wrote:
> 
> > 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));
> 
> I tried something similar. It did not work, because than the hint is 
> "bound" to the closest next call in the function which is cr4_init() in 
> this case. The unwinder would not take it into account.
> 
> In my case, I placed it at the beginning of cpu_bringup_and_idle(). I also 
> open coded it and played with the offset in the orc entry, but that did 
> not work for some other reason.
> 
> However, now I tried this
> 
> diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> index 6b88cdcbef8f..39afd88309cb 100644
> --- a/arch/x86/xen/smp_pv.c
> +++ b/arch/x86/xen/smp_pv.c
> @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
>  {
>         cpu_bringup();
>         boot_init_stack_canary();
> +       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
>         cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
>  }
> 
> and that seems to work. I need to properly verify and test, but the 
> explanation is that as opposed to the above, cpu_startup_entry() is on the 
> idle task's stack and the hint is then taken into account. The unwound 
> stack seems to be complete, so it could indeed be the fix.

Not the correct one though. Objtool rightfully complains with

arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state

and all the other hacks I tried ended up in the same dead alley. It seems 
to me the correct fix is that all orc entries for cpu_bringup_and_idle() 
should have "end" property set to 1, since it is the first function on the 
stack. I don't know how to achieve that without the assembly hack in the 
patch I sent. If I am not missing something, of course.

Josh, any idea?

Thanks
Miroslav

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

[Josh Poimboeuf]

On Mon, Mar 16, 2020 at 04:51:12PM +0100, Miroslav Benes wrote:
> > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> > index 6b88cdcbef8f..39afd88309cb 100644
> > --- a/arch/x86/xen/smp_pv.c
> > +++ b/arch/x86/xen/smp_pv.c
> > @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
> >  {
> >         cpu_bringup();
> >         boot_init_stack_canary();
> > +       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
> >         cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> >  }
> > 
> > and that seems to work. I need to properly verify and test, but the 
> > explanation is that as opposed to the above, cpu_startup_entry() is on the 
> > idle task's stack and the hint is then taken into account. The unwound 
> > stack seems to be complete, so it could indeed be the fix.
> 
> Not the correct one though. Objtool rightfully complains with
> 
> arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state
> 
> and all the other hacks I tried ended up in the same dead alley. It seems 
> to me the correct fix is that all orc entries for cpu_bringup_and_idle() 
> should have "end" property set to 1, since it is the first function on the 
> stack. I don't know how to achieve that without the assembly hack in the 
> patch I sent. If I am not missing something, of course.
> 
> Josh, any idea?

Yeah, I think mucking with the unwind hints in C code is going to be
precarious.  You could maybe have something like

	asm("
	  UNWIND_HINT_EMPTY\n
	  mov $CPUHP_AP_ONLINE_IDLE, %rdi\n
	  call cpu_startup_entry\n
	)"
	unreachable();

but that's pretty ugly (and it might not work anyway).

I suppose we could add a new facility to mark an entire C function as an
"end" point.

But I think it would be cleanest to just do something like your patch
and have the entry code be asm which then calls cpu_bringup_and_idle().
That would make it consistent with all other entry code, which all lives
in asm.

-- 
Josh

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

[Miroslav Benes]

On Mon, 16 Mar 2020, Boris Ostrovsky wrote:

> 
> 
> On 3/12/20 10:20 AM, Miroslav Benes wrote:
> > --- 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
> 
> This is initial_stack, isn't it?

It is. I'll change it.

Thanks
Miroslav

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

[Miroslav Benes]

On Mon, 16 Mar 2020, Josh Poimboeuf wrote:

> On Mon, Mar 16, 2020 at 04:51:12PM +0100, Miroslav Benes wrote:
> > > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> > > index 6b88cdcbef8f..39afd88309cb 100644
> > > --- a/arch/x86/xen/smp_pv.c
> > > +++ b/arch/x86/xen/smp_pv.c
> > > @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
> > >  {
> > >         cpu_bringup();
> > >         boot_init_stack_canary();
> > > +       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
> > >         cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> > >  }
> > > 
> > > and that seems to work. I need to properly verify and test, but the 
> > > explanation is that as opposed to the above, cpu_startup_entry() is on the 
> > > idle task's stack and the hint is then taken into account. The unwound 
> > > stack seems to be complete, so it could indeed be the fix.
> > 
> > Not the correct one though. Objtool rightfully complains with
> > 
> > arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state
> > 
> > and all the other hacks I tried ended up in the same dead alley. It seems 
> > to me the correct fix is that all orc entries for cpu_bringup_and_idle() 
> > should have "end" property set to 1, since it is the first function on the 
> > stack. I don't know how to achieve that without the assembly hack in the 
> > patch I sent. If I am not missing something, of course.
> > 
> > Josh, any idea?
> 
> Yeah, I think mucking with the unwind hints in C code is going to be
> precarious.  You could maybe have something like
> 
> 	asm("
> 	  UNWIND_HINT_EMPTY\n
> 	  mov $CPUHP_AP_ONLINE_IDLE, %rdi\n
> 	  call cpu_startup_entry\n
> 	)"
> 	unreachable();
> 
> but that's pretty ugly (and it might not work anyway).
> 
> I suppose we could add a new facility to mark an entire C function as an
> "end" point.

I think it would be an overkill for what I perceive as one-off scenario. 
Maybe if there are more use cases in the future, but I doubt it.
 
> But I think it would be cleanest to just do something like your patch
> and have the entry code be asm which then calls cpu_bringup_and_idle().
> That would make it consistent with all other entry code, which all lives
> in asm.

Ack.

Thanks
Miroslav