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

[Vitaly Kuznetsov]

Zhao Liu <zhao1.liu@intel.com> writes:

> 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.

This is about QEMU/KVM emulating certain Hyper-V version, not about
guest Hyper-V version. To be honest, I'm not sure what was the last
version of Hyper-V which was exposing HV_SYSTEM_RESET_RECOMMENDED. I
don't have anything older that WS2016 around now and the bit is not
there. If I'm not mistaken, it was already missing in 2012R2. I would
appreciate if anyone has more precise historical info to add here.

>
>> +- ``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.

Np, if I am to re-send this I'll add these (hope it's not an acceptance
blocker, we can always do a follow-up).

>
> 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?

Hyper-V is a role you can enable in various Windows versions, both
server and client. When enabled, you get a hypervisor (which is called
'Microsoft Hypervisor' as I was told) and your Windows becomes the root
partition (similar to Xen Dom0). In case you run this on KVM, Windows
becomes L2. Hyper-V enlightenments provided by KVM/QEMU are consumed by
the hypervisor then.

Note: Hyper-V role is optional, in many cases Windows guests run without
it (no Hyper-V VMs, no WSL2, ...) and thus consume KVM's Hyper-V
enlightenments directly, no nested virt involved.

-- 
Vitaly