linux-kernel - Re: [PATCH v5 03/26] x86/hyperv: Update 'struct hv_enlightened

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Message-ID: <Yv59dZwP6rNUtsrn@google.com>
Date:   Thu, 18 Aug 2022 17:57:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 03/26] x86/hyperv: Update 'struct hv_enlightened_vmcs'
 definition

On Thu, Aug 18, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > On Tue, Aug 02, 2022, Vitaly Kuznetsov wrote:
> >> + * Note: HV_X64_NESTED_EVMCS1_2022_UPDATE is not currently documented in any
> >> + * published TLFS version. When the bit is set, nested hypervisor can use
> >> + * 'updated' eVMCSv1 specification (perf_global_ctrl, s_cet, ssp, lbr_ctl,
> >> + * encls_exiting_bitmap, tsc_multiplier fields which were missing in 2016
> >> + * specification).
> >> + */
> >> +#define HV_X64_NESTED_EVMCS1_2022_UPDATE		BIT(0)
> >
> > This bit is now defined[*], but the docs says it's only for perf_global_ctrl.  Are
> > we expecting an update to the TLFS?
> >
> > 	Indicates support for the GuestPerfGlobalCtrl and HostPerfGlobalCtrl fields
> > 	in the enlightened VMCS.
> >
> > [*] https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/feature-discovery#hypervisor-nested-virtualization-features---0x4000000a
> >
> 
> Oh well, better this than nothing. I'll ping the people who told me
> about this bit that their description is incomplete.

Not that it changes anything, but I'd rather have no documentation.  I'd much rather
KVM say "this is the undocumented behavior" than "the document behavior is wrong".