Re: [PATCH v4 00/19] i386: KVM: expand Hyper-V features early and provide simple 'hv-default=on' option

[Eduardo Habkost]

On Wed, Feb 10, 2021 at 04:56:06PM +0000, Daniel P. Berrangé wrote:
> 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 ?

"all currently supported" in this context means "all features
supported when the machine type was added", not "all features
supported by the host kernel".

-- 
Eduardo