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

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Message-ID: <ZU5F58_KRIHzxrMp@google.com>
Date:   Fri, 10 Nov 2023 07:01:59 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xin3 Li <xin3.li@...el.com>
Cc:     Chao Gao <chao.gao@...el.com>,
        "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>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls
 in vmcs_config

On Fri, Nov 10, 2023, Xin3 Li wrote:
> > > >+	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.

No, remove it.  It's architecturally legal for a CPU to support a feature in bare
metal but not provide virtualization support.

> > 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.

Uh, no.  This is not optional.  FRED is the first feature that uses SECONDAY_VM_EXIT
controls, so FRED gets the honor of adding the necessary infrastructure support.