Re: [PATCH 2/2] Expose tsc deadline timer cpuid to guest

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-01-05 18:07, Liu, Jinsong wrote:
>> Sorry, it remains bogus to expose the tsc deadline timer feature on
>> machines < pc-1.1. That's just like we introduced kvmclock only to
>> pc-0.14 onward. The reason is that guest OSes so far running on
>> qemu-1.0 or older without deadline timer support must not find that
>> feature when being migrated to a host with qemu-1.1 in pc-1.0 compat
>> mode. Yes, the user can explicitly disable it, but that is not the
>> idea of legacy machine models. They should provide the very same
>> environment that older qemu versions offered.
>>
> 
> Not quite clear about this point.
> Per my understanding, if a kvm guest running on an older qemu without tsc deadline timer support, 
> then after migrate, the guest would still cannot find tsc deadline feature, no matter older or newer host/qemu/pc-xx it migrate to.

What should prevent this? The feature flags are not part of the vmstate.
They are part of the vm configuration which is not migrated but defined
by starting qemu on the target host.

Jan


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