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

[Liu, Jinsong]

Jan Kiszka wrote:
> 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.
> 

Thanks! understand this point ("They are part of the vm configuration which is 
not migrated but defined by starting qemu on the target host").

But kvmclock example still cannot satisfy the purpose "guest running on old 
qemu/pc-0.13 without kvmclock support must not find kvmclock feature when being 
migrated to a host with new qemu/pc-0.13 compat mode". After migration, guest 
can possibily find kvmclock feature CPUID.0x40000001.KVM_FEATURE_CLOCKSOURCE:
pc_init1(..., kvmclock_enabled) 
{
    pc_cpus_init(cpu_model);    // the point to decide and expose cpuid 
features to guest

    if (kvmclock_enabled) {        // the difference point between pc-0.13 vs. 
pc-0.14, related nothing to cpuid features.
        kvmclock_create();
    }
}

Seems currently there is no good way to satisfy "guest running on old 
qemu/pc-xx without feature A support must not find feature A when being 
migrated to a host with new qemu/pc-xx compat mode", i.e. considering
* if running with '-cpu host' then migrate;
* each time we add a new cpuid feature it need add one or more new machine 
model? is it necessary to bind pc-xx with cpuid feature?
* logically cpuid features should better be controlled by cpu model, not by 
machine model.

Regards,
Jinsong