linux-kernel - RE: [PATCH v1 15/23] KVM: nVMX: Add support for the secondary VM exit controls

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Date:   Fri, 10 Nov 2023 00:12:53 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.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>
CC:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "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>,
        "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>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v1 15/23] KVM: nVMX: Add support for the secondary VM exit
 controls

> > diff --git a/arch/x86/include/asm/hyperv-tlfs.h
> > b/arch/x86/include/asm/hyperv-tlfs.h
> > index 2ff26f53cd62..299554708e37 100644
> > --- a/arch/x86/include/asm/hyperv-tlfs.h
> > +++ b/arch/x86/include/asm/hyperv-tlfs.h
> > @@ -616,6 +616,7 @@ struct hv_enlightened_vmcs {
> >  	u64 host_ssp;
> >  	u64 host_ia32_int_ssp_table_addr;
> >  	u64 padding64_6;
> > +	u64 secondary_vm_exit_controls;
> >  } __packed;
> > >  #define HV_VMX_ENLIGHTENED_CLEAN_FIELD_NONE			0
> 
> Hi Xin Li,
> 
> The comment at the top of hyperv-tlfs.h says:
> "This file contains definitions from Hyper-V Hypervisor Top-Level Functional
> Specification (TLFS)"
> 
> These struct definitions are shared with the hypervisor, so you can't just add fields
> to it.
> Same comment for patch 16.

I tried not to touch any hyperv stuff but hit some problems.

> 
> Would FRED work in nested virtualization if the L0 hypervisor does not support it
> (or doesn't know about it)?

I don't think so AFAICT.  But I could be wrong, say a VMM implements FRED
completely in software.  Otherwise L0 needs to add code to have hardware
to switch FRED context between host and guest, which can't be delayed.

Thanks a lot for your quick review!