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Re: [PATCH 5/5] i386: provide simple 'hyperv=on' option to x86 machine t

From: Vitaly Kuznetsov
Subject: Re: [PATCH 5/5] i386: provide simple 'hyperv=on' option to x86 machine types
Date: Wed, 06 Jan 2021 14:38:56 +0100

Igor Mammedov <imammedo@redhat.com> writes:

> On Tue, 05 Jan 2021 17:31:43 +0100
> Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> Igor Mammedov <imammedo@redhat.com> writes:
>> > On Tue, 05 Jan 2021 12:50:05 +0100
>> >
>> > I think there is a misunderstanding, idea was:
>> >
>> > cpu_initfn() {
>> >     //current set
>> >     cpu->default_hyperv_cpu_features = ACD
>> > }
>> >
>> > compat_props_5.1 {
>> >    cpu.default_hyperv_cpu_features = AB
>> > }
>> >
>> > compat_props_5.2 {
>> >    cpu.default_hyperv_cpu_features = ABC
>> > }
>> >  
>> ...
>> > I was talking about CPU features/properties only, it doesn't apply to 
>> > other devices.
>> > It makes sense for machine to have a knob to create onboard hyperv specific
>> > devices if there is any (do we have any?).
>> >
>> > If there aren't any currently, I wouldn't bother with machine knob
>> > and just use -cpu foo,hv_default=on or -device cpu,hv_default=on
>> > like any other cpu feature.
>> >  
>> We don't currently have any devices which are not 'CPU features' (in
>> QEMU terminology), however, we already have Vmbus and I can easily
>> imagine us implementing e.g. hartbeat/kvp/vss/... devices on top. We
>> *may* want to enable these 'automatically' and that's what make
>> '-machine' option preferable. It is, however, not a *must* right now and
>> we can indeed wait until these devices appear and be happy with
>> 'hv_default' -cpu option for now. We will, however, need to teach upper
>> layers about the change when/if it happens.
> which makes me think we are trying to bite something that we shouldn't.
> Do we really need this patch (QEMU knob) to magically enable subset of
> features and/or devices for a specific OS flavor?
> It's job of upper layers to abstract low level QEMU details in to coarse
> grained knobs (libvirt/virt-install/virt-manager/...).
> For example virt-install may know that it installing a specific Windows
> version, and can build a tailored for that OS configuration including
> needed hyperv CPU features and hyperv specific devices.
> (if I'm not mistaken libosinfo is used to get metadata for preferred
> configuration, so perhaps this should become a patch for that library
> and its direct users).
> What we actually lack is a documentation for preferred configuration
> in docs/hyperv.txt, currently it just enumerates possible features.
> We can just document a recommended 'best practices' there without
> putting it in QEMU code and let upper layers to do their job in
> the stack.

The problem we're facing here is that when a new enlightenment is
implemented it takes forever to propagate to the whole stack. We don't
have any different recommendations for different Windows versions,
neither does genuine Hyper-V. The 'fine grained' mechanis we have just
contributes to the creation of various Frankenstein configurations
(which look nothing like real Hyper-V), people just google for 'Windows
KVM slow', add something to their scripts and this keeps propagating.

Every time I see a configuration with only a few 'hv_*' options I ask
'why don't you enable the rest?' and I'm yet to receive an answer
different from 'hm, I don't know, I copied it from somewhere and it

Setting 'hv_*' options individually should be considered debug only.


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