linux-kernel - Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Message-ID: <ZkdpKiSyOwB3NwRD@google.com>
Date: Fri, 17 May 2024 14:26:50 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Weijiang Yang <weijiang.yang@...el.com>, rick.p.edgecombe@...el.com, 
	pbonzini@...hat.com, dave.hansen@...el.com, x86@...nel.org, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterz@...radead.org, 
	chao.gao@...el.com, mlevitsk@...hat.com, john.allen@....com
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On Fri, May 17, 2024, Thomas Gleixner wrote:
> 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.  With
> >> this policy, it can avoid CPUID confusion to guest side due to host ibt=off
> >> 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 still show
> > IBT support in the host.
> >
> > On the other hand, I can definitely see folks wanting to expose IBT to guests
> > when running non-complaint host kernels, especially when live migration is 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.

Ya, I don't disagree, I just didn't realize that CET_USER would be cleared in the
supported xfeatures mask.