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

[Eduardo Habkost]

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).

-- 
Eduardo