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Date: Tue, 30 Apr 2024 09:13:28 -0700
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Message-ID: <ZjEYqCgtt4ZgVKth@google.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module
From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: Tina Zhang <tina.zhang@intel.com>, Hang Yuan <hang.yuan@intel.com>, 
	Bo2 Chen <chen.bo@intel.com>, "sagis@google.com" <sagis@google.com>, 
	"isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Erdem Aktas <erdemaktas@google.com>, 
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	Isaku Yamahata <isaku.yamahata@intel.com>, 
	"isaku.yamahata@linux.intel.com" <isaku.yamahata@linux.intel.com>
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On Tue, Apr 30, 2024, Kai Huang wrote:
> On 30/04/2024 8:06 am, Sean Christopherson wrote:
> > My suggestion is essentially "throw in a CR4.VMXE check before
> > TDH.SYS.LP.INIT if it's easy".  If it's not easy for some reason, then don't do
> > it.
> 
> I see.  The disconnection between us is I am not super clear why we should
> treat TDH.SYS.LP.INIT as a special one that deserves a CR4.VMXE check but
> not other SEAMCALLs.

Because TDH.SYS.LP.INIT is done on all CPUs via an IPI function call, is a one-
time thing, and is at the intersection of core TDX and KVM module code, e.g. the
the core TDX code has an explicit assumption that:

 * This function assumes the caller has: 1) held read lock of CPU hotplug
 * lock to prevent any new cpu from becoming online; 2) done both VMXON
 * and tdx_cpu_enable() on all online cpus.

KVM can obviously screw up and attempt SEAMCALLs without being post-VMXON, but
that's entirely a _KVM_ bug.  And the probability of getting all the way to
something like TDH_MEM_SEPT_ADD without being post-VMXON is comically low, e.g.
KVM and/or the kernel would likely crash long before that point.