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

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Message-ID: <SA1PR11MB6734FBE99A7FB30FF4085B7CA8B2A@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Tue, 14 Nov 2023 04:05:24 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     "Gao, Chao" <chao.gao@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
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        "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

> > > > 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.

Like the stage when native Linux has FRED support while KVM not yet?

> > > 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.

The 2nd VM exit controls is a must for FRED, so I should do it.

I think you mean the consistency checks can be done in a better way (which
is not just for FRED controls consistency checks).  No?

Thanks!
    Xin