linux-kernel - Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

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Date:   Fri, 4 Aug 2023 23:45:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Chao Gao <chao.gao@...el.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, peterz@...radead.org,
        john.allen@....com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com,
        binbin.wu@...ux.intel.com
Subject: Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

On 8/4/23 23:27, Sean Christopherson wrote:
>>> +
>>> +	if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
>>> +	    !kvm_cpu_cap_has(X86_FEATURE_IBT))
>>> +		return false;
>>> +
>>> +	return msr->host_initiated ||
>>> +		guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
>>> +		guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>
> Similar to my suggestsion for XSS, I think we drop the waiver for host_initiated
> accesses, i.e. require the feature to be enabled and exposed to the guest, even
> for the host.

No, please don't.  Allowing host-initiated accesses is what makes it 
possible to take the list of MSR indices and pass it blindly to 
KVM_GET_MSR and KVM_SET_MSR.  This should be documented, will send a patch.

Paolo