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Date: Thu, 25 Apr 2024 09:30:59 -0700
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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 23, 2024, Kai Huang wrote:
> On Tue, 2024-04-23 at 22:59 +0000, Huang, Kai wrote:
> > > Right, but that doesn't say why the #UD occurred.=C2=A0 The macro dre=
sses it up in
> > > TDX_SW_ERROR so that KVM only needs a single parser, but at the end o=
f the day
> > > KVM is still only going to see that SEAMCALL hit a #UD.
> >=20
> > Right.=C2=A0 But is there any problem here?=C2=A0 I thought the point w=
as we can
> > just use the error code to tell what went wrong.
>=20
> Oh, I guess I was replying too quickly.  From the spec, #UD happens when
>=20
> 	IF not in VMX operation or inSMM or inSEAM or=C2=A0
> 			((IA32_EFER.LMA & CS.L) =3D=3D 0)
>  		THEN #UD;
>=20
> Are you worried about #UD was caused by other cases rather than "not in
> VMX operation"?

Yes.
=20
> But it's quite obvious the other 3 cases are not possible, correct?

The spec I'm looking at also has:

	If IA32_VMX_PROCBASED_CTLS3[5] is 0.

And anecdotally, I know of at least one crash in our production environment=
 where
a VMX instruction hit a seemingly spurious #UD, i.e. it's not impossible fo=
r a
ucode bug or hardware defect to cause problems.  That's obviously _extremel=
y_
unlikely, but that's why I emphasized that sanity checking CR4.VMXE is chea=
p.
Practically speaking it costs nothing, so IMO it's worth adding even if the=
 odds
of it ever being helpful are one-in-and-million.