Re: [PATCHv5.1 14/16] x86/smp: Add smp_ops.stop_this_cpu() callback

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Mon, Jan 08, 2024 at 03:04:31AM +0000, Huang, Kai wrote:
> On Mon, 2023-12-25 at 11:05 +0300, Kirill A. Shutemov wrote:
> > If the helper is defined, it is called instead of halt() to stop the CPU
> > at the end of stop_this_cpu() and on crash CPU shutdown.
> > 
> > ACPI MADT will use it to hand over the CPU to BIOS in order to be able
> > to wake it up again after kexec.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> > 
> >  v5.1:
> >    - Fix build for !SMP;
> > 
> > ---
> >  arch/x86/include/asm/smp.h |  1 +
> >  arch/x86/kernel/process.c  |  7 +++++++
> >  arch/x86/kernel/reboot.c   | 12 ++++++++----
> >  3 files changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> > index 4fab2ed454f3..390d53fd34f9 100644
> > --- a/arch/x86/include/asm/smp.h
> > +++ b/arch/x86/include/asm/smp.h
> > @@ -38,6 +38,7 @@ struct smp_ops {
> >  	int (*cpu_disable)(void);
> >  	void (*cpu_die)(unsigned int cpu);
> >  	void (*play_dead)(void);
> > +	void (*stop_this_cpu)(void);
> >  
> >  	void (*send_call_func_ipi)(const struct cpumask *mask);
> >  	void (*send_call_func_single_ipi)(int cpu);
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index b6f4e8399fca..ea4c812c7bf3 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -835,6 +835,13 @@ void __noreturn stop_this_cpu(void *dummy)
> >  	 */
> >  	cpumask_clear_cpu(cpu, &cpus_stop_mask);
> >  
> > +#ifdef CONFIG_SMP
> > +	if (smp_ops.stop_this_cpu) {
> > +		smp_ops.stop_this_cpu();
> > +		unreachable();
> > +	}
> > +#endif
> 
> If I read correctly this will result in stop_this_cpu() having different
> behaviour for SMP and !SMP build for TDX guest.  For example, AFAICT
> machine_halt() also calls stop_this_cpu() on local cpu after it stops other
> cpus.  So for the local cpu, in SMP build it will calls into BIOS's reset vector
> but in !SMP it will call native_halt().

It doesn't make a difference in practice: both halt and giving CPU to
BIOS will be unrecoverable operation. Both are equally acceptable for
machine_halt().

> > +
> >  	for (;;) {
> >  		/*
> >  		 * Use native_halt() so that memory contents don't change
> > diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> > index 16dde83df49a..738b3e810196 100644
> > --- a/arch/x86/kernel/reboot.c
> > +++ b/arch/x86/kernel/reboot.c
> > @@ -881,10 +881,14 @@ static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
> >  	cpu_emergency_disable_virtualization();
> >  
> >  	atomic_dec(&waiting_for_crash_ipi);
> > -	/* Assume hlt works */
> > -	halt();
> > -	for (;;)
> > -		cpu_relax();
> > +
> > +	if (smp_ops.stop_this_cpu) {
> > +		smp_ops.stop_this_cpu();
> 
> Could you explain why unreachable() is called in stop_this_cpu() but not here?

Compiler complained previously on stop_this_cpu() when it had halt() in
'else' case because the function is declared as __noreturn. I left
unreachable() after reworking it without 'else' to document the behaviour.

> > +	} else {
> > +		halt();
> > +		for (;;)
> > +			cpu_relax();
> > +	}
> 
> Similar to stop_this_cpu(), if you also call unreachable() here, then I think
> you can remove the 'else' here but directly calls halt() + cpu_relax() loop.

It doesn't make much difference to me. But okay, I will rework it the same
way in the next version.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov