linux-kernel - Re: [PATCH v16 19/51] KVM: x86: Don't emulate task switches when IBT or SHSTK is enabled

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Message-ID: <aNEyt/z8eAXQr8/r@intel.com>
Date: Mon, 22 Sep 2025 19:27:51 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <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>, Binbin Wu
	<binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>, "Maxim
 Levitsky" <mlevitsk@...hat.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>,
	"Xin Li" <xin@...or.com>
Subject: Re: [PATCH v16 19/51] KVM: x86: Don't emulate task switches when IBT
 or SHSTK is enabled

>@@ -12178,6 +12178,25 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
> 	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
> 	int ret;
> 
>+	if (kvm_is_cr4_bit_set(vcpu, X86_CR4_CET)) {
>+		u64 u_cet, s_cet;
>+
>+		/*
>+		 * Check both User and Supervisor on task switches as inter-
>+		 * privilege level task switches are impacted by CET at both
>+		 * the current privilege level and the new privilege level, and
>+		 * that information is not known at this time.  The expectation
>+		 * is that the guest won't require emulation of task switches
>+		 * while using IBT or Shadow Stacks.
>+		 */
>+		if (__kvm_emulate_msr_read(vcpu, MSR_IA32_U_CET, &u_cet) ||
>+		    __kvm_emulate_msr_read(vcpu, MSR_IA32_S_CET, &s_cet))
>+			return EMULATION_FAILED;

is it ok to return EMULATION_FAILED (-1) here?

It looks like this error code will be propagated to userspace and be
interpreted as -EPERM.