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

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Date: Fri, 22 Mar 2024 16:06:42 +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>, "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>
Subject: Re: [PATCH v19 130/130] RFC: KVM: x86, TDX: Add check for
 KVM_SET_CPUID2

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 read that Sean's point was
that KVM_SET_CPUID2 can't accept anything today what we would want to
block later, otherwise it would introduce a regression. This was the
major constraint IIUC, and means the base series requires *something*
here.

If we want to support only the most basic support first, we don't need
to support PV CPUIDs on day 1, right?

So I'm wondering, if we could shrink the base series by going with
option 1 to start, and then expanding it with this solution later to
enable more features. Do you see a problem or conflict with Sean's
comments?