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

[Liu, Jinsong]

Liu, Jinsong wrote:
> Jan Kiszka wrote:
>> On 2012-03-06 08:49, Liu, Jinsong wrote:
>>> Jan,
>>> 
>>> Any comments? I feel some confused about your point 'disable cpuid
>>> feature for older machine types by default': are you planning a
>>> common approach for this common issue, or, you just ask me a
>>> specific solution for the tsc deadline timer case?
>> 
>> I think a generic solution for this can be as simple as passing a
>> feature exclusion mask to cpu_init. You could simple append a string
>> of "-feature1,-feature2" to the cpu model that is specified on
>> creation. And that string could be defined in the compat machine
>> descriptions. Does this make sense?
>> 
> 
> Jan, to prevent misunderstanding, I elaborate my understanding of
> your points below (if any misunderstanding please point out to me):
> ===================== 
> Your target is, to migrate from A(old qemu) to B(new qemu) by
> 1. at A: qemu-version-A [-cpu whatever]      // currently the default
> machine type is pc-A 
> 2. at B: qemu-version-B -machine pc-A [-cpu whatever] -feature1
> -feature2 
> 
> B run new qemu-version-B (w/ new features 'feature1' and 'feature2'),
> but when B runs w/ compat '-machine pc-A', vm should not see
> 'feature1' and 'feature2', so commandline append string to cpu model
> '-cpu whatever -feature1 -feature2' to hidden new feature1 and
> feature2 to vm, hence vm can see same cpuid features (at B) as those
> at A (which means, no feature1, no feature2) =====================    
> 

BTW, any misunderstanding or something wrong about my understanding of your 
target? please help me confirm. I want to make sure we are talking same thing. 

Thanks,
Jinsong

> If my understanding of your thoughts is right, I think currently qemu
>      has satisfied your target, code refer to pc_cpus_init(cpu_model)
>      ......
>      cpu_init(cpu_model)
> --> cpu_x86_register(*env, cpu_model)
> --> cpu_x86_find_by_name(*def, cpu_model)     // parse '+/-
>                                                                     
> features', generate feature masks plus_features... // and
> minus_features...(this is feature exclusion masks you want)  
> I think your point 'define in the compat machine description' is
> unnecessary. 
> 
> As for 'tsc deadline' feature exposing, my patch (as attached) just
> obey qemu general cpuid exposing method, and also satisfied your
> target I think.  
> 
> 
> Thanks,
> Jinsong