linux-kernel - Re: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for KVM_SET

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Date: Mon, 25 Mar 2024 22:37:50 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Huang, Kai"
	<kai.huang@...el.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "Chen, Bo2" <chen.bo@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for
 KVM_SET_CPUID2

On Mon, 2024-03-25 at 22:31 +0000, Huang, Kai wrote:
> (sorry replying from outlook due to some issue to my linux box environment)
> 
> It booted because Qemu does sane thing, i.e., it always passes the correct CPUIDs in
> KVM_SET_CPUID2.
> 
> Per-Sean's comments, KVM should guarantee the consistency between CPUIDs done in TDH.MNG.INIT and
> KVM_SET_CPUID2, otherwise if Qemu passes in-consistent CPUIDs KVM can easily fail to work with TD.
> 
> To guarantee the consistency, KVM could do two options as we discussed:
> 
> 1) reject KVM_SET_CPUID2 completely.
> 2) Still allow KVM_SET_CPUID2 but manually check the CPUID consistency between the one done in
> TDH.MNG.INIT and the one passed in KVM_SET_CPUID2.
> 
> 1) can obviously guarantee consistency.  But KVM maintains CPUIDs in 'vcpu', so to make the
> existing KVM code continue to work, we need to manually set 'vcpu->cpuid' to the one that is done
> in TDH.MNG.INIT. 
> 
> 2) you need to check the consistency and reject KVM_SET_CPUID2 if in-consistency found.  But other
> than that, KVM doesn't need to anything more because if we allow KVM_SET_CPUID2, the 'vcpu' will
> have its own CPUIDs populated anyway.

Ah, thanks for explaining. So 1 is not that simple, it is a maybe slightly smaller separate
solution. Now I see why the discussion was to just do the consistency checking up front.