linux-kernel - Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on demand

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Date:   Fri, 30 Jun 2023 14:58:02 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Kai Huang <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Rafael J Wysocki" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>, Sagi Shahar <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, Chao Gao <chao.gao@...el.com>,
        Len Brown <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ying Huang <ying.huang@...el.com>,
        Dan J Williams <dan.j.williams@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand

Sean Christopherson wrote:
> On Fri, Jun 30, 2023, Isaku Yamahata wrote:
> > On Fri, Jun 30, 2023 at 08:30:20PM +0200,
> > Peter Zijlstra <peterz@...radead.org> wrote:
[..]
> On the flip side, limited hardware availability (unless Intel has changed its
> tune) and the amount of enabling that's required in BIOS and whatnot makes it
> highly unlikely that random Linux users are going to unknowingly boot with TDX
> enabled.
> 
> That said, if this is a sticking point, let's just make enable_tdx off by default,
> i.e. force userspace to opt-in.  Deployments that *know* they may want to schedule
> TDX VMs on the host can simply force the module param.  And for everyone else,
> since KVM is typically configured as a module by distros, KVM can be unloaded and
> reload if the user realizes they want TDX well after the system is up and running.

Another potential option that also avoids the concern that module
parameters are unwieldy [1] is to have kvm_intel have a soft-dependency
on something like a kvm_intel_tdx module. That affords both a BIOS *and*
userspace policy opt-out where kvm_intel.ko can check that
kvm_intel_tdx.ko is present at init time, or proceed with tdx disabled.

[1]: http://lore.kernel.org/r/Y7z99mf1M5edxV4A@kroah.com