linux-kernel - Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

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Message-ID: <ZiqFQ1OSFM4OER3g@google.com>
Date: Thu, 25 Apr 2024 09:30:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Tina Zhang <tina.zhang@...el.com>, Hang Yuan <hang.yuan@...el.com>, 
	Bo2 Chen <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Erdem Aktas <erdemaktas@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module

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.  The macro dresses it up in
> > > TDX_SW_ERROR so that KVM only needs a single parser, but at the end of the day
> > > KVM is still only going to see that SEAMCALL hit a #UD.
> > 
> > Right.  But is there any problem here?  I thought the point was we can
> > just use the error code to tell what went wrong.
> 
> Oh, I guess I was replying too quickly.  From the spec, #UD happens when
> 
> 	IF not in VMX operation or inSMM or inSEAM or 
> 			((IA32_EFER.LMA & CS.L) == 0)
>  		THEN #UD;
> 
> Are you worried about #UD was caused by other cases rather than "not in
> VMX operation"?

Yes.
 
> 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 for a
ucode bug or hardware defect to cause problems.  That's obviously _extremely_
unlikely, but that's why I emphasized that sanity checking CR4.VMXE is cheap.
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.