linux-kernel - Re: [PATCH v16 28/51] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Message-ID: <5717b2dd-8a95-4200-a547-b724f46abaef@intel.com>
Date: Tue, 23 Sep 2025 22:57:41 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Tom Lendacky <thomas.lendacky@....com>,
 Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>,
 Binbin Wu <binbin.wu@...ux.intel.com>, Maxim Levitsky <mlevitsk@...hat.com>,
 Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: Re: [PATCH v16 28/51] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@...el.com>
> 
> Add support for the LOAD_CET_STATE VM-Enter and VM-Exit controls, the
> CET XFEATURE bits in XSS, and  advertise support for IBT and SHSTK to
> userspace.  Explicitly clear IBT and SHSTK onn SVM, as additional work is
> needed to enable CET on SVM, e.g. to context switch S_CET and other state.
> 
> Disable KVM CET feature if unrestricted_guest is unsupported/disabled as
> KVM does not support emulating CET, as running without Unrestricted Guest
> can result in KVM emulating large swaths of guest code.  While it's highly
> unlikely any guest will trigger emulation while also utilizing IBT or
> SHSTK, there's zero reason to allow CET without Unrestricted Guest as that
> combination should only be possible when explicitly disabling
> unrestricted_guest for testing purposes.
> 
> Disable CET if VMX_BASIC[bit56] == 0, i.e. if hardware strictly enforces
> the presence of an Error Code based on exception vector, as attempting to
> inject a #CP with an Error Code (#CP architecturally has an Error Code)
> will fail due to the #CP vector historically not having an Error Code.
> 
> Clear S_CET and SSP-related VMCS on "reset" to emulate the architectural
> of CET MSRs and SSP being reset to 0 after RESET, power-up and INIT.  Note,
> KVM already clears guest CET state that is managed via XSTATE in
> kvm_xstate_reset().
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> [sean: move some bits to separate patches, massage changelog]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>