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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: Mon, 28 Jan 2019 18:22:30 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

* Vitaly Kuznetsov (address@hidden) wrote:
> Roman Kagan <address@hidden> writes:
> > On Fri, Jan 25, 2019 at 02:46:42PM +0100, Vitaly Kuznetsov wrote:
> >> Roman Kagan <address@hidden> writes:
> >> 
> >> > On Fri, Jan 25, 2019 at 12:41:51PM +0100, Vitaly Kuznetsov wrote:
> >> >> In many case we just want to give Windows guests all currently supported
> >> >> Hyper-V enlightenments and that's where this new mode may come handy. We
> >> >> pass through what was returned by KVM_GET_SUPPORTED_HV_CPUID.
> >> >
> >> > How is the compatibility ensured on migration between kernels reporting
> >> > different feature sets?
> >> 
> >> AFAIU we don't change anything in this regard (or, my intention was to
> >> not change anything): hv-all is converted to the individual hv-*
> >> properties (hv_cpuid_check_and_set()) actually sets cpu->hyperv_* flags
> >> according to what's supported by kernel so when we migrate we will
> >> require all these features supported.
> >
> > Migration relies on the upper layer to run the destination QEMU with the
> > identical command line (except for -incoming) as the source, and QEMU is
> > then supposed to set up identical environment in the target VM as was in
> > the source, or refuse to start if that's impossible.  (If I'm
> > misunderstanding this Dave (cc-d) may want to correct me.)
> >
> > AFAICS this hv-all attribute will enable different feature sets
> > depending on the kernel it's run on, so the migration between different
> > kernels will appear to succeed, but the guest may suddenly encounter an
> > incompatible change in the environment.
> With 'hv-all' I'm trying to achieve behavior similar to '-cpu host' and
> AFAIK these VMs are migratable 'at your own risk' (if you do it directly
> from qemu). Libvirt (or whatever upper layer), however, would do CPU
> feature comparison and in case you have less features on the destination
> host than you had on the source code it will forbid the migration. I
> think if this also works for Hyper-V features than were fine.
> Dave, feel free to tell me I'm completely wrong with my assumptions)

It does sound like -cpu host, but -cpu host does come with a health
warning and we often get subtle screwups where it doesn't quite behave
the same on the two sides, also qemu now warns (and with 'enforce'
enforces) a check at it's level rather than relying on libvirt.

So hmm, yes it sounds like -cpu host, but I'd generally say it's not a
great thing to copy unless you're really really careful.
For example, in the -cpu host world people might have two machines
they think are the same - but then they find out one has HT disabled
or nesting enabled and so they're not actually the same.

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?

Also, how does libvirt figure out that the features are actually the
same - does it need a bunch of detection code?


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

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