Re: [PATCH RFC 07/11] KVM: x86: Make Hyper-V emulation optional

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Mon, Oct 16, 2023, Vitaly Kuznetsov wrote:
>> Maxim Levitsky <mlevitsk@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx/

"""
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.
"""

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.

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.

-- 
Vitaly