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 18:54:20 -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 Fri, Mar 22, 2024 at 07:10:42AM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> On Thu, 2024-03-21 at 23:12 +0000, Edgecombe, Rick P wrote:
> > On Mon, 2024-02-26 at 00:27 -0800, isaku.yamahata@...el.com wrote:
> > > Implement a hook of KVM_SET_CPUID2 for additional consistency check.
> > > 
> > > Intel TDX or AMD SEV has a restriction on the value of cpuid.  For
> > > example,
> > > some values must be the same between all vcpus.  Check if the new
> > > values
> > > are consistent with the old values.  The check is light because the
> > > cpuid
> > > consistency is very model specific and complicated.  The user space
> > > VMM
> > > should set cpuid and MSRs consistently.
> > 
> > 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.
> 
> 
> Btw, Isaku, I don't understand why you tag the last two patches as RFC and put
> them at last.  I think I've expressed this before.  Per the discussion with
> Sean, my understanding is this isn't something optional but the right thing we
> should do?
> 
> https://lore.kernel.org/lkml/ZDiGpCkXOcCm074O@google.com/

Ok, let's remove RFC and reorder this patches.  Do you see any issue of the
cpuid check logic itself?
-- 
Isaku Yamahata <isaku.yamahata@...el.com>