Re: [PATCH RESEND v3 3/3] docs/system: Add recommendations to Hyper-V enlightenments doc

[Zhao Liu]

Hi Vitaly,

On Tue, Mar 05, 2024 at 05:42:04PM +0100, Vitaly Kuznetsov wrote:
> Date: Tue,  5 Mar 2024 17:42:04 +0100
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Subject: [PATCH RESEND v3 3/3] docs/system: Add recommendations to Hyper-V
>  enlightenments doc
> 
> While hyperv.rst already has all currently implemented Hyper-V
> enlightenments documented, it may be unclear what is the recommended set to
> achieve the best result. Add the corresponding section to the doc.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  docs/system/i386/hyperv.rst | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/docs/system/i386/hyperv.rst b/docs/system/i386/hyperv.rst
> index 009947e39141..1c1de77feb65 100644
> --- a/docs/system/i386/hyperv.rst
> +++ b/docs/system/i386/hyperv.rst
> @@ -283,6 +283,36 @@ Supplementary features
>    feature alters this behavior and only allows the guest to use exposed 
> Hyper-V
>    enlightenments.
>  
> +Recommendations
> +---------------

This guide is very helpful!

> +To achieve the best performance of Windows and Hyper-V guests and unless 
> there
> +are any specific requirements (e.g. migration to older QEMU/KVM versions,
> +emulating specific Hyper-V version, ...), it is recommended to enable all
> +currently implemented Hyper-V enlightenments with the following exceptions:
> +
> +- ``hv-syndbg``, ``hv-passthrough``, ``hv-enforce-cpuid`` should not be 
> enabled
> +  in production configurations as these are debugging/development features.
> +- ``hv-reset`` can be avoided as modern Hyper-V versions don't expose it.

Does the "Hyper-V versions" means Hyper-V guest version or Microsoft's Hyper-V
hypervisor version? It would be better to clarify Hyper-V guest and
Hyper-v hypervisor.

And it would be better to have a clear version number.

> +- ``hv-evmcs`` can (and should) be enabled on Intel CPUs only. While the 
> feature
> +  is only used in nested configurations (Hyper-V, WSL2), enabling it for 
> regular
> +  Windows guests should not have any negative effects.
> +- ``hv-no-nonarch-coresharing`` must only be enabled if vCPUs are properly 
> pinned
> +  so no non-architectural core sharing is possible.
> +- ``hv-vendor-id``, ``hv-version-id-build``, ``hv-version-id-major``,
> +  ``hv-version-id-minor``, ``hv-version-id-spack``, 
> ``hv-version-id-sbranch``,
> +  ``hv-version-id-snumber`` can be left unchanged, guests are not supposed to
> +  behave differently when different Hyper-V version is presented to them.
> +- ``hv-crash`` must only be enabled if the crash information is consumed via
> +  QAPI by higher levels of the virtualization stack. Enabling this feature
> +  effectively prevents Windows from creating dumps upon crashes.
> +- ``hv-reenlightenment`` can only be used on hardware which supports TSC
> +  scaling or when guest migration is not needed.
> +- ``hv-spinlocks`` should be set to e.g. 0xfff when host CPUs are 
> overcommited
> +  (meaning there are other scheduled tasks or guests) and can be left 
> unchanged
> +  from the default value (0xffffffff) otherwise.
> +- ``hv-avic``/``hv-apicv`` should not be enabled if the hardware does not
> +  support APIC virtualization (Intel APICv, AMD AVIC).
>

It's also better to add blank lines between paragraphs above.

BTW, may I ask another Windows question? I understand that Windows such
as Windows 10 and later is already a virtualized architecture with
built-in Hyper-V to run root partation.

So is it true that booting Windows VM via KVM + QEMU is running Windows
Guest in L2? Or what is the relationship between Hyper-V within Windows
and Hyper-V enlightenments with QEMU + KVM?

Thanks,
Zhao