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

["Huang, Kai" <kai.huang@intel.com>]

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@linux.intel.com>
> ---
> 
>  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().

> +
>  	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?

> +	} 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.