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Re: [Qemu-devel] [PATCH RFC 4/8] i386/kvm: Implement 'hv-all' pass-throu

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH RFC 4/8] i386/kvm: Implement 'hv-all' pass-through mode
Date: Tue, 29 Jan 2019 15:28:12 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

* Vitaly Kuznetsov (address@hidden) wrote:
> "Dr. David Alan Gilbert" <address@hidden> writes:
> > I'm not sure what the equivalent bear traps are in the Hyper-V world,
> > but I'd be surprised if there weren't any; for example what happens
> > when someone upgrades one of their hosts to some minor version that
> > adds/removes a feature?
> Here we're talking about Hyper-V emulation in KVM, features only get
> added there, but even if it gets removed it will be detected by libvirt ...

OK, but then you do get the same behaviour; upgrade a host to a new
kernel/qemu and get a new enlightenment, and you can't back migrate to
the older one (possibly with no warning).

> >
> > Also, how does libvirt figure out that the features are actually the
> > same - does it need a bunch of detection code?
> ... as I *think* it compares Feature CPUID words (and all Hyper-V
> features which we enable with hv-all are there).

Not too bad if it does, but also look at the scary command lines we get
generated full of -cpu ...+feature,+feature,-feature,.... 


> -- 
> Vitaly
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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