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

[Liu, Jinsong]

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)
=====================

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

0001-Expose-tsc-deadline-timer-feature-to-guest.patch
Description: 0001-Expose-tsc-deadline-timer-feature-to-guest.patch