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Date:   Mon, 16 Oct 2023 09:45:39 -0700
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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:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Mon, Oct 16, 2023, Vitaly Kuznetsov wrote:
> >> Maxim Levitsky <mlevitsk@redhat.com> writes:
> >> 
> 
> ...
> 
> >> >
> >> > "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 documentation
> > that calls it "Microsoft Hypervisor", i.e. Hyper-V is the full and
> > proper name.
> 
> Ha :-) From
> https://lore.kernel.org/linux-hyperv/1696010501-24584-1-git-send-email-nunodasneves@linux.microsoft.com/
> 
> """
> This series introduces support for creating and running guest machines
> while running on the Microsoft Hypervisor. [0]
> ...
> [0] "Hyper-V" is more well-known, but it really refers to the whole stack
>     including the hypervisor and other components that run in Windows
>     kernel and userspace.
> """

LOL indeed.  :-)

> I'm fine with keeping the staus quo though :-)
> 
> >
> >> > This makes KVM expose a set of paravirtualized interfaces,
> >
> > s/makes/allows, since KVM still requires userspace to opt-in to exposing Hyper-V.
> >
> >> > documented in the HyperV TLFS, 
> >
> > s/TLFS/spec?  Readers that aren't already familiar with Hyper-V will have no idea
> > what TLFS is until they click the link.
> >
> >> > https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs,
> >> > which consists of a subset of paravirtualized interfaces that HyperV exposes
> >
> > We can trim this paragraph by stating that KVM only supports a subset of the
> > PV interfaces straightaway.
> >
> >> > to its guests.
> >
> > E.g.
> >
> >   Provides KVM support for for emulating Microsoft Hyper-V.  This allows KVM to
> >   expose a subset of the paravirtualized interfaces defined in Hyper-V's spec:
> >   https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs.

To fold in the whole "Microsoft Hypervisor" thing, what if we take the verbiage
verbatim from the TLFS intro?

  Provides KVM support for emulating Microsoft Hyper-V.  This allows KVM to
  expose a subset of the paravirtualized interfaces defined in the Hyper-V
  Hypervisor Top-Level Functional Specification (TLFS):
  https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs


> LGTM, thanks!
> 
> >
> >> >
> >> > This improves performance of modern Windows guests.
> >
> > Isn't Hyper-V emulation effectively mandatory these days?  IIRC, modern versions
> > of Windows will fail to boot if they detect a hypervisor but the core Hyper-V
> > interfaces aren't supported.
> >
> 
> It's rather a rule of thumb: normally, modern Windows and Hyper-V
> versions (Win10/11, WS2019/22) boot and pretend to work but without
> Hyper-V enlightenment it's not uncommon to see a blue screen of death
> because of a watchdog firing. It's hard to say for sure as things keep
> changing under the hood so even different builds can behave differently;
> pretending we're a genuine Hyper-V was proven to be the most robust
> approach.

We should capture something to that effect in the help.  I want to avoid bug
reports of Windows not working on KVM because someone turned off CONFIG_KVM_HYPERV
because the help implies that it _only_ improves performance.