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

[Liu, Jinsong]

Eduardo Habkost wrote:
> On Fri, Mar 23, 2012 at 03:49:27AM +0000, Liu, Jinsong wrote:
>> Eduardo Habkost wrote:
>>> [1] From Documentation/virtual/kvm/api.txt:
>>> 
>>> "KVM_GET_SUPPORTED_CPUID
>>> [...]
>>> This ioctl returns x86 cpuid features which are supported by both
>>> the hardware and kvm.  Userspace can use the information returned
>>> by this ioctl to construct cpuid information (for KVM_SET_CPUID2)
>>> that is consistent with hardware, kernel, and userspace
>>>                                   capabilities, and with
>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^ 
>>> user requirements (for example, the user may wish to constrain cpuid
>>> to emulate older hardware, or for feature consistency across a
>>> cluster)."
>> 
>> The fixbug patch is implemented by Jan and Avi, I reply per my
>> understanding. 
> 
> No problem. I hope Jan or Avi can clarify this.
> 
>> 
>> I think for tsc deadline timer feature, KVM_CAP_TSC_DEADLINE_TIMER is
>> slightly better than KVM_GET_SUPPORTED_CPUID. If use
>> KVM_GET_SUPPORTED_CPUID, it means tsc deadline features bind to host
>> cpuid, while it fact it could be pure software emulated by kvm
>> (though currently it implemented as bound to hareware). For the sake
>> of 
>> extension, it choose KVM_CAP_TSC_DEADLINE_TIMER.
> 
> There's no requirement for GET_SUPPORTED_CPUID to be a subset of the
> host CPU features. If KVM can completely emulate the feature by
> software, then it can return the feature on GET_SUPPORTED_CPUID even
> if the host CPU doesn't have the feature. That's the case for x2apic,
> for example (see commit 0d1de2d901f4ba0972a3886496a44fb1d3300dbd).


Jan/Avi,

Could you elaborate more your thought? 

Thanks,
Jinsong