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Re: [PATCH v4 00/19] i386: KVM: expand Hyper-V features early and provid

From: Daniel P . Berrangé
Subject: Re: [PATCH v4 00/19] i386: KVM: expand Hyper-V features early and provide simple 'hv-default=on' option
Date: Wed, 10 Feb 2021 16:56:06 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Wed, Feb 10, 2021 at 05:40:12PM +0100, Vitaly Kuznetsov wrote:
> Changes since v3:
> - Make 'hv-default' override 'hv-*' options which were already set 
>   (e.g. 'hv-feature=on,hv-default' case) [Igor]. Make 'hv-passthrough'
>   behave the same way.
> - Add "i386: be more picky about implicit 'hv-evmcs' enablement" patch to 
> avoid
>   enabling 'hv-evmcs' with hv-default/hv-passthrough when guest CPU lacks VMX.
> - Add "i386: support 'hv-passthrough,hv-feature=off' on the command line" 
> patch
>   to make 'hv-passthrough' semantics match the newly introduced 'hv-default'.
> - Add "i386: track explicit 'hv-*' features enablement/disablement" patch to
>   support the above mentioned changes.
> - Expand qtest to check the above mentioned improvements.
> Original description:
> Upper layer tools like libvirt want to figure out which Hyper-V features are
> supported by the underlying stack (QEMU/KVM) but currently they are unable to
> do so. We have a nice 'hv_passthrough' CPU flag supported by QEMU but it has
> no effect on e.g. QMP's 
> query-cpu-model-expansion type=full 
> model={"name":"host","props":{"hv-passthrough":true}}
> command as we parse Hyper-V features after creating KVM vCPUs and not at
> feature expansion time. To support the use-case we first need to make 
> KVM_GET_SUPPORTED_HV_CPUID ioctl a system-wide ioctl as the existing
> vCPU version can't be used that early. This is what KVM part does. With
> that done, we can make early Hyper-V feature expansion (this series).
> In addition, provide a simple 'hv-default' option which enables (and
> requires from KVM) all currently supported Hyper-V enlightenments.
> Unlike 'hv-passthrough' mode, this is going to be migratable.

How is it going to be migratable if the semantics vary depending on
the host kernel KVM reporting features, because different kernels
will expose different features ?

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