From:   Thomas Gleixner <tglx@linutronix.de>
To:     Usama Arif <usama.arif@bytedance.com>, dwmw2@infradead.org,
        kim.phillips@amd.com, brgerst@gmail.com
Cc:     piotrgorski@cachyos.org, oleksandr@natalenko.name,
        arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org,
        pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com,
        David Woodhouse <dwmw@amazon.co.uk>,
        Usama Arif <usama.arif@bytedance.com>
Subject: Re: [PATCH v12 06/11] x86/smpboot: Remove initial_stack on 64-bit
In-Reply-To: <20230226110802.103134-7-usama.arif@bytedance.com>
References: <20230226110802.103134-1-usama.arif@bytedance.com>
 <20230226110802.103134-7-usama.arif@bytedance.com>
Date:   Tue, 28 Feb 2023 17:13:55 +0100
Message-ID: <87k001n4xo.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Sun, Feb 26 2023 at 11:07, Usama Arif wrote:
> From: Brian Gerst <brgerst@gmail.com>
>
> Eliminating global variables from the CPU startup path in order to simplify
> it and facilitate parallel startup.

As this patch is now part of the parallel boot series and actually
introduces smpboot_control, the above is neither accurate nor useful.

Folks, really.

> Remove initial_stack, and load RSP from current_task->thread.sp instead.

  
>  #ifdef CONFIG_SMP
> -	initial_stack = (unsigned long)temp_stack + sizeof(temp_stack);
> +	current->thread.sp = (unsigned long)temp_stack + sizeof(temp_stack);

This lacks a comment about the temporary (ab)use of current->thread.sp

>  	early_gdt_descr.address =
>  			(unsigned long)get_cpu_gdt_rw(smp_processor_id());
>  	initial_gs = per_cpu_offset(smp_processor_id());
> +	smpboot_control = smp_processor_id();
>  #endif

> @@ -241,6 +241,24 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>  	UNWIND_HINT_EMPTY
>  	ANNOTATE_NOENDBR // above
>  
> +#ifdef CONFIG_SMP
> +	movl	smpboot_control(%rip), %ecx
> +
> +	/* Get the per cpu offset for the given CPU# which is in ECX */
> +	movq	__per_cpu_offset(,%rcx,8), %rdx
> +#else
> +	xorl	%edx, %edx
> +#endif /* CONFIG_SMP */

Sigh, we should finally make CONFIG_SMP def_bool y ...

> +	/*
> +	 * Setup a boot time stack - Any secondary CPU will have lost its stack
> +	 * by now because the cr3-switch above unmaps the real-mode stack.
> +	 *
> +	 * RDX contains the per-cpu offset
> +	 */
> +	movq	pcpu_hot + X86_current_task(%rdx), %rax
> +	movq	TASK_threadsp(%rax), %rsp
> +
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index b18c1385e181..62e3bf37f0b8 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1112,7 +1112,12 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
>  	idle->thread.sp = (unsigned long)task_pt_regs(idle);
>  	early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(cpu);
>  	initial_code = (unsigned long)start_secondary;
> -	initial_stack  = idle->thread.sp;
> +
> +	if (IS_ENABLED(CONFIG_X86_32)) {
> +		initial_stack  = idle->thread.sp;
> +	} else {
> +		smpboot_control = cpu;
> +	}

Please remove the pointless brackets.
  
Thanks,

        tglx