linux-kernel - Re: [PATCH v9 04/18] x86/virt/tdx: Add skeleton to initialize TDX on demand

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Date:   Tue, 14 Feb 2023 09:23:40 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Kai Huang <kai.huang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, tglx@...utronix.de, seanjc@...gle.com,
        pbonzini@...hat.com, dan.j.williams@...el.com,
        rafael.j.wysocki@...el.com, kirill.shutemov@...ux.intel.com,
        ying.huang@...el.com, reinette.chatre@...el.com,
        len.brown@...el.com, tony.luck@...el.com, ak@...ux.intel.com,
        isaku.yamahata@...el.com, chao.gao@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, david@...hat.com,
        bagasdotme@...il.com, sagis@...gle.com, imammedo@...hat.com
Subject: Re: [PATCH v9 04/18] x86/virt/tdx: Add skeleton to initialize TDX on
 demand

On 2/14/23 04:46, Peter Zijlstra wrote:
> On Tue, Feb 14, 2023 at 12:59:11AM +1300, Kai Huang wrote:
>> Use a state machine protected by mutex to make sure the initialization
>> will only be done once, as tdx_enable() can be called multiple times
>> (i.e. KVM module can be reloaded) and be called concurrently by other
>> kernel components in the future.
> I still object to doing tdx_enable() at kvm module load.
> 
> kvm.ko gets loaded unconditionally on boot, even if I then never use
> kvm.
> 
> This stuff needs to be done when an actual VM is created, not before.

The actually implementation of this is hidden over in the KVM side of
this.  But, tdx_enable() and all of this jazz should not be called on
kvm.ko load.  It'll happen when the KVM tries to start the first TDX VM.

I think what Kai was thinking of was *this* sequence:

 1. insmod kvm.ko
 2. Start a TDX guest, tdx_enable() gets run
 3. rmmod kvm
 4. insmod kvm.ko (again)
 5. Start another TDX guest, run tdx_enable() (again)

The rmmod/insmod pair is what triggers the second call of tdx_enable().