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 14:10:33 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"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>,
	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 11:14:21AM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> On Fri, 2024-03-22 at 16:06 +0000, Edgecombe, Rick P wrote:
> > On Fri, 2024-03-22 at 07:10 +0000, Huang, Kai wrote:
> > > > I see that this was suggested by Sean, but can you explain the
> > > > problem
> > > > that this is working around? From the linked thread, it seems like
> > > > the
> > > > problem is what to do when userspace also calls SET_CPUID after
> > > > already
> > > > configuring CPUID to the TDX module in the special way. The choices
> > > > discussed included:
> > > > 1. Reject the call
> > > > 2. Check the consistency between the first CPUID configuration and
> > > > the
> > > > second one.
> > > > 
> > > > 1 is a lot simpler, but the reasoning for 2 is because "some KVM
> > > > code
> > > > paths rely on guest CPUID configuration" it seems. Is this a
> > > > hypothetical or real issue? Which code paths are problematic for
> > > > TDX/SNP?
> > > 
> > > There might be use case that TDX guest wants to use some CPUID which
> > > isn't handled by the TDX module but purely by KVM.  These (PV) CPUIDs
> > > need to be
> > > provided via KVM_SET_CPUID2.
> > 
> > Right, but are there any needed today? 
> > 
> 
> I am not sure.  Isaku may know better?

It's not needed to boot TD.  The check is safe guard.  The multiple of source of
cpuids can be inconsistent.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>