linux-kernel - Re: [PATCH v7 3/6] x86/topology: Disable CPU online/offline control for TDX guest

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Date:   Sun, 17 Oct 2021 21:23:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/6] x86/topology: Disable CPU online/offline control
 for TDX guest

On Tue, Oct 05 2021 at 16:05, Kuppuswamy Sathyanarayanan wrote:
>  
> +static int tdx_cpu_offline_prepare(unsigned int cpu)
> +{
> +	/*
> +	 * Per Intel TDX Virtual Firmware Design Guide,
> +	 * sec 4.3.5 and sec 9.4, Hotplug is not supported
> +	 * in TDX platforms. So don't support CPU
> +	 * offline feature once it is turned on.
> +	 */
> +	return -EOPNOTSUPP;
> +}
> +
>  unsigned long tdx_get_ve_info(struct ve_info *ve)
>  {
>  	struct tdx_module_output out = {0};
> @@ -451,5 +464,8 @@ void __init tdx_early_init(void)
>  	pv_ops.irq.safe_halt = tdx_safe_halt;
>  	pv_ops.irq.halt = tdx_halt;
>  
> +	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "tdx:cpu_hotplug",
> +			  NULL, tdx_cpu_offline_prepare);

Seriously? This lets the unplug start, which starts to kick off tasks
from the CPU just to make it fail a few steps later?

The obvious place to prevent this is the CPU hotplug code itself, right?

Thanks,

        tglx
---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 192e43a87407..c544eb6c79d3 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1178,6 +1178,8 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
 
 static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
 {
+	if (cc_platform_has(CC_HOTPLUG_DISABLED))
+		return -ENOTSUPP;
 	if (cpu_hotplug_disabled)
 		return -EBUSY;
 	return _cpu_down(cpu, 0, target);