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From: Thomas Gleixner <tglx@linutronix.de>
To: Sean Christopherson <seanjc@google.com>, Weijiang Yang
 <weijiang.yang@intel.com>
Cc: rick.p.edgecombe@intel.com, pbonzini@redhat.com, dave.hansen@intel.com,
 x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
 peterz@infradead.org, chao.gao@intel.com, mlevitsk@redhat.com,
 john.allen@amd.com
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX
 and advertise to userspace
In-Reply-To: <ZkYauRJBhaw9P1A_@google.com>
References: <20240219074733.122080-1-weijiang.yang@intel.com>
 <20240219074733.122080-25-weijiang.yang@intel.com>
 <ZjLNEPwXwPFJ5HJ3@google.com>
 <39b95ac6-f163-4461-93f3-eaa653ab1355@intel.com>
 <ZkYauRJBhaw9P1A_@google.com>
Date: Fri, 17 May 2024 10:57:03 +0200
Message-ID: <87r0e0ke8w.ffs@tglx>
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On Thu, May 16 2024 at 07:39, Sean Christopherson wrote:
> On Thu, May 16, 2024, Weijiang Yang wrote:
>> We synced the issue internally, and got conclusion that KVM should honor=
 host
>> IBT config.  In this case IBT bit in boot_cpu_data should be honored.=C2=
=A0 With
>> this policy, it can avoid CPUID confusion to guest side due to host ibt=
=3Doff
>> config.
>
> What was the reasoning?  CPUID confusion is a weak justification, e.g. it=
's not
> like the guest has visibility into the host kernel, and raw CPUID will st=
ill show
> IBT support in the host.
>
> On the other hand, I can definitely see folks wanting to expose IBT to gu=
ests
> when running non-complaint host kernels, especially when live migration i=
s in
> play, i.e. when hiding IBT from the guest will actively cause problems.

I have to disagree here violently.

If the exposure of a CPUID bit to a guest requires host side support,
e.g. in xstate handling, then exposing it to a guest is simply not
possible.

Just because virtualization allows to do that does not mean that it's
correct in any way.

Thanks,

        tglx