linux-kernel - Re: [PATCH v2 21/21] KVM:x86: Support CET supervisor shadow stack MSR access

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Message-ID: <175c826d59f1ac77fa588908d3768ffc2e79268e.camel@intel.com>
Date:   Wed, 3 May 2023 17:07:15 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "john.allen@....com" <john.allen@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 21/21] KVM:x86: Support CET supervisor shadow stack MSR
 access

On Fri, 2023-04-21 at 09:46 -0400, Yang Weijiang wrote:
> +
> +       incpt = !is_cet_state_supported(vcpu,
> XFEATURE_MASK_CET_KERNEL);
> +       incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> +
> +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> MSR_TYPE_RW, incpt);
> +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> MSR_TYPE_RW, incpt);
> +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> MSR_TYPE_RW, incpt);
> +       vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> MSR_TYPE_RW, incpt);
>  }

Why is this tied to XFEATURE_MASK_CET_KERNEL? I don't know how the SVM
side works, but the host kernel doesn't use this xfeature. Just not
clear on what the intention is. Why not use
kvm_cet_kernel_shstk_supported() again?