Re: [Xen-devel] [PATCH 5/7] xen/arm: Remove __initdata and __init to enable CPU hotplug

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 12/04/18 13:50, Mirela Simonovic wrote:
> Hi,
>
> On Thu, Apr 12, 2018 at 11:03 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
> > Hi,
> >
> > On 12/04/18 01:07, Stefano Stabellini wrote:
> > > On Wed, 11 Apr 2018, Mirela Simonovic wrote:
> > > > diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
> > > > index 5666efcd3a..d15ea8df5e 100644
> > > > --- a/xen/arch/arm/smpboot.c
> > > > +++ b/xen/arch/arm/smpboot.c
> > > > @@ -52,8 +52,8 @@ nodemask_t __read_mostly node_online_map = { { [0] =
> > > > 1UL } };
> > > >    static unsigned char __initdata cpu0_boot_stack[STACK_SIZE]
> > > >           __attribute__((__aligned__(STACK_SIZE)));
> > > >    -/* Initial boot cpu data */
> > > > -struct init_info __initdata init_data =
> > > > +/* Boot cpu data */
> > > > +struct init_info init_data =
> > > >    {
> > > >        .stack = cpu0_boot_stack,
> > > >    };
> > >
> > > Don't you also want to remove __initdata from cpu0_boot_stack?
> Somehow I didn't observe this as a problem... After taking a deeper
> look now I understand that secondary CPUs reuse this stack to boot. So
> I agree, __initdata from cpu0_boot_stack should be removed.
No it should not be removed. cpu0_boot_stack is only used for Xen is 
booted (e.g CPU0 jumping at _start). In the suspend/resume case you are 
not going to use that patch for CPU0.
> > I am not sure about this. When you go idle, you could re-use the
> > idle_vcpu[0]->arch.stack. So you save 12K in resident memory.
> I'm not sure I follow this, maybe Stefano can comment.
Each CPU have an associated idle vCPU used for context switch and 
running idle mode. That idle vCPU contains the stack that is used for 
boot CPU.
In the case of CPU0, you can not use idle vCPU stack when booting 
because it is not initialized. However during suspend/resume case, you 
will already have the idle_vcpu[0]->stack in hand. So there are no need 
to use cpu0_boot_stack.
However, do you really need to setup the stack on resume?

Cheers,

--
Julien Grall

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