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 14:17:23 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "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>,
	"Zhang, Tina" <tina.zhang@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"Yuan, Hang" <hang.yuan@...el.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>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for
 KVM_SET_CPUID2

On Mon, Mar 25, 2024 at 03:32:59PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Mon, 2024-03-25 at 11:14 +0000, Huang, Kai wrote:
> > To confirm, I mean you want to simply make KVM_SET_CPUID2 return error for TDX
> > guest?
> > 
> > It is acceptable to me, and I don't see any conflict with Sean's comments.
> > 
> > But I don't know Sean's perference.  As he said, I think  the consistency
> > checking is quite straight-forward:
> > 
> > "
> > It's not complicated at all.  Walk through the leafs defined during
> > TDH.MNG.INIT, reject KVM_SET_CPUID if a leaf isn't present or doesn't match
> > exactly.
> > "
> > 
> Yea, I'm just thinking if we could take two patches down to one small one it might be a way to
> essentially break off this work to another series without affecting the ability to boot a TD. It
> *seems* to be the way things are going.
> 
> > So to me it's not a big deal. 
> > 
> > Either way, we need a patch to handle SET_CPUID2:
> > 
> > 1) if we go option 1) -- that is reject SET_CPUID2 completely -- we need to make
> > vcpu's CPUID point to KVM's saved CPUID during TDH.MNG.INIT.
> 
> Ah, I missed this part. Can you elaborate? By dropping these two patches it doesn't prevent a TD
> boot. If we then reject SET_CPUID, this will break things unless we make other changes? And they are
> not small?

If we go forthis, the extended topology enumeration (cpuid[0xb or 0x1f]) would
need special handling because it's per-vcpu. not TD wide.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>