Re: [PATCH v2] KVM: x86: Prevent exposing TSC deadline timer feature in the absence of in-kernel APIC

[Avi Kivity <avi@redhat.com>]

On 12/26/2011 10:11 AM, Liu, Jinsong wrote:
> > 
> > It breaks live migration: if you start a guest on a TSC-deadline
> > capable host kernel, and migrate it to a TSC-deadline incapable host
> > kernel, you end up with a broken guest.
> > 
> > More broadly, kvm never exposes features transparently to the guest,
> > it always passes them to userspace first, so userspace controls the
> > ABI exposed to the guest.  This prevents the following scenario:
>
> Do you mean, by the method qemu control cpuid exposing, it can avoid live migration broken issue by
> 1. user probe the lowest ability host of whole pool where vm may live migrate;
> 2. only if the lowest ablility host support the feature can user enable the feature when boot a vm;
> 3. if the lowest ability host didn't support the feature (say tsc deadline timer as example), user disable the feature when boot a vm;
> In this way, live migration wouldn't be broken. Right?

Right.

> or, do you mean qemu-kvm solve live migration broken issue by some other method?

The method you outlined, or any other method, such as partitioning the
cluster according to hardware capabilities.

>
> > 
> > - a guest is started on some hardware, which doesn't support some
> > cpuid feature (say AVX for example)
> > - the guest or one of its applications are broken wrt AVX, but because
> > the feature is not exposed, it works correctly
> > - the host hardware is upgraded to one which supports AVX
> > - the guest is now broken
>
> You mean, live migrate from 'old' (which doesn't support the feature) platform to 'new' platform would broken?

Live migration, or even just a guest restart on updated hardware.

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