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Date: Tue, 7 May 2024 07:21:28 -0700
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Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace
From: Sean Christopherson <seanjc@google.com>
To: Rick P Edgecombe <rick.p.edgecombe@intel.com>
Cc: Chao Gao <chao.gao@intel.com>, Dave Hansen <dave.hansen@intel.com>, 
	"x86@kernel.org" <x86@kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	"peterz@infradead.org" <peterz@infradead.org>, "john.allen@amd.com" <john.allen@amd.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Weijiang Yang <weijiang.yang@intel.com>, 
	"mlevitsk@redhat.com" <mlevitsk@redhat.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>
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On Mon, May 06, 2024, Rick P Edgecombe wrote:
> I don't immediately see what trouble will be in giving kernel IBT a disable
> parameter that doesn't touch X86_FEATURE_IBT at some point in the future.

Keeping X86_FEATURE_IBT set will result in "ibt" being reported in /proc/cpuinfo,
i.e. will mislead userspace into thinking IBT is supported and fully enabled by
the kernel.  For a security feature, that's a pretty big issue.

To fudge around that, we could add a synthetic feature flag to let the kernel
tell KVM whether or not it's safe to virtualize IBT, but I don't see what value
that adds over KVM checking raw host CPUID.