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Date:   Mon, 16 Oct 2023 08:27:01 -0700
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Message-ID: <ZS1VGvbcmH93-KyH@google.com>
Subject: Re: [PATCH RFC 07/11] KVM: x86: Make Hyper-V emulation optional
From:   Sean Christopherson <seanjc@google.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        linux-kernel@vger.kernel.org
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On Mon, Oct 16, 2023, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@redhat.com> writes:
>=20
> > =D0=A3 =D0=B2=D1=82, 2023-10-10 =D1=83 18:02 +0200, Vitaly Kuznetsov =
=D0=BF=D0=B8=D1=88=D0=B5:
> >> Hyper-V emulation in KVM is a fairly big chunk and in some cases it ma=
y be
> >> desirable to not compile it in to reduce module sizes as well as attac=
k
> >> surface. Introduce CONFIG_KVM_HYPERV option to make it possible.
> >>=20
> >> Note, there's room for further nVMX/nSVM code optimizations when
> >> !CONFIG_KVM_HYPERV, this will be done in follow-up patches.
> >
> > Maybe CONFIG_KVM_HYPERV_GUEST_SUPPORT or CONFIG_HYPERV_ON_KVM instead?
> >
> > IMHO CONFIG_KVM_HYPERV_GUEST_SUPPORT sounds good.

Adding GUEST_SUPPORT doesn't disambiguate anything though, as there's no cl=
ear
indication of whether KVM or Hyper-V is the guest.  E.g. the umbrella kconf=
ig for
Linux-as-a-guest is CONFIG_HYPERVISOR_GUEST.

> We already have CONFIG_KVM_XEN so I decided to stay concise. I do
> understand that 'KVM-on-Hyper-V' and 'Hyper-V-on-KVM' mess which creates
> the confusion though.

Yeah, matching Xen is probably the best way to minimize confusion, e.g. the=
 kernel
has CONFIG_HYPERV and CONFIG_XEN to go with KVM's, CONFIG_KVM_HYPERV and CO=
NFIG_KVM_XEN.

> >> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> >> index ed90f148140d..a06e19a8a8f6 100644
> >> --- a/arch/x86/kvm/Kconfig
> >> +++ b/arch/x86/kvm/Kconfig
> >> @@ -129,6 +129,15 @@ config KVM_SMM
> >> =20
> >>  	  If unsure, say Y.
> >> =20
> >> +config KVM_HYPERV
> >> +	bool "Support for Microsoft Hyper-V emulation"
> >> +	depends on KVM
> >> +	default y
> >> +	help
> >> +	  Provides KVM support for emulating Microsoft Hypervisor (Hyper-V).
> >
> >
> > It feels to me that the KConfig option can have a longer description.
> >
> > What do you think about something like that:
> >
> > "Provides KVM support for emulating Microsoft Hypervisor (Hyper-V).

I don't think we should put Hyper-V in parentheses, I haven't seen any docu=
mentation
that calls it "Microsoft Hypervisor", i.e. Hyper-V is the full and proper n=
ame.

> > This makes KVM expose a set of paravirtualized interfaces,

s/makes/allows, since KVM still requires userspace to opt-in to exposing Hy=
per-V.

> > documented in the HyperV TLFS,=20

s/TLFS/spec?  Readers that aren't already familiar with Hyper-V will have n=
o idea
what TLFS is until they click the link.

> > https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/refe=
rence/tlfs,
> > which consists of a subset of paravirtualized interfaces that HyperV ex=
poses

We can trim this paragraph by stating that KVM only supports a subset of th=
e
PV interfaces straightaway.

> > to its guests.

E.g.

  Provides KVM support for for emulating Microsoft Hyper-V.  This allows KV=
M to
  expose a subset of the paravirtualized interfaces defined in Hyper-V's sp=
ec:
  https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/refere=
nce/tlfs.

> >
> > This improves performance of modern Windows guests.

Isn't Hyper-V emulation effectively mandatory these days?  IIRC, modern ver=
sions
of Windows will fail to boot if they detect a hypervisor but the core Hyper=
-V
interfaces aren't supported.