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Date:   Fri, 30 Jun 2023 14:58:02 -0700
From:   Dan Williams <dan.j.williams@intel.com>
To:     Sean Christopherson <seanjc@google.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
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        <sathyanarayanan.kuppuswamy@linux.intel.com>,
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Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand
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Sean Christopherson wrote:
> On Fri, Jun 30, 2023, Isaku Yamahata wrote:
> > On Fri, Jun 30, 2023 at 08:30:20PM +0200,
> > Peter Zijlstra <peterz@infradead.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