linux-kernel - Re: [PATCH v9 07/18] x86/virt/tdx: Do TDX module per-cpu initialization

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Message-ID: <2d7141b1-1d76-4e67-60d2-471a524c372e@intel.com>
Date:   Mon, 13 Feb 2023 14:28:43 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>
Subject: Re: [PATCH v9 07/18] x86/virt/tdx: Do TDX module per-cpu
 initialization

On 2/13/23 13:13, Huang, Kai wrote:
> Perhaps I didn't explain clearly in the comment.  Below is the updated one:
> 
>                 /*
>                  * The previous call of __tdx_enable() may only have
>                  * initialized part of present cpus during module
>                  * initialization, and new cpus may have become online
>                  * since then w/o doing per-cpu initialization.
>                  *
>                  * For example, a new CPU can become online when KVM is
>                  * unloaded, in which case tdx_cpu_enable() is not called since
>                  * KVM's CPU online callback has been removed.
>                  *
>                  * To make sure all online cpus are TDX-runnable, always
>                  * do per-cpu initialization for all online cpus here
>                  * even the module has been initialized.
>                  */

This is voodoo.

I want a TDX-specific hotplug CPU handler.  Period.  Please make that
happen.  Put that code in this patch.  That handler should:

	1. Run after the KVM handler (if present)
	2. See if VMX is on
	3. If VMX is on:
	 3a. Run smp_func_module_lp_init(), else
	 3b. Mark the CPU as needing smp_func_module_lp_init()

Then, in the 'case TDX_MODULE_INITIALIZED:', you call a function to
iterate over the cpumask that was generated in 3b.

That makes the handoff *EXPLICIT*.  You know exactly which CPUs need
what done to them.  A CPU hotplug either explicitly involves doing the
work to make TDX work on the CPU, or explicitly defers the work to a
specific later time in a specific later piece of code.