linux-kernel - RE: [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls in vmcs

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Date:   Fri, 10 Nov 2023 00:04:16 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Gao, Chao" <chao.gao@...el.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "Cui, Dexuan" <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls
 in vmcs_config

> > >+	if (cpu_feature_enabled(X86_FEATURE_FRED) &&
> > >+	    !(_vmentry_control & VM_ENTRY_LOAD_IA32_FRED)) {
> > >+		pr_warn_once("FRED enabled but no VMX VM-Entry
> LOAD_IA32_FRED control: %x\n",
> > >+			     _vmentry_control);
> >
> > Can we just hide FRED from guests like what KVM does for other
> > features which have similar dependencies? see vmx_set_cpu_caps().
> 
> Both of these warnings should simply be dropped.  The
> error_on_inconsistent_vmcs_config stuff is for inconsistencies within the allowed
> VMCS fields.  Having a feature that is supported in bare metal but not virtualized
> is perfectly legal, if uncommon.

I deliberately keep it, at least for now, because these checks are helpful
during the development of nVMX FRED.  It will be helpful for other VMMs
being developed/tested on KVM.

> What *is* needed is for KVM to refuse to virtualize FRED if the entry/exit controls
> aren't consistent.  E.g. if at least one control is present, and at least one
> control is missing.   I.e. KVM needs a version of vmcs_entry_exit_pairs that can
> deal with SECONDAY_VM_EXIT controls.

I agree there are better ways.  But maybe after or before VMX FRED.

>  I'll circle back to this when I give the
> series a proper review, which is going to be 3+ weeks.

The traffic in KVM mailing list is surprisingly high recently.  So that is
totally expected.