RE: [PATCH v2] KVM: x86: Prevent exposing TSC deadline timerfeature in the absence of in-kernel APIC

[Liu, Jinsong]

Avi Kivity wrote:
> On 12/21/2011 12:25 PM, Jan Kiszka wrote:
>> We must not report the TSC deadline timer feature on our own when
>> user space provides the APIC as we have no clue about its features.
> 
> We must not report the TSC deadline timer feature on our own, period.
> We should just update the timer mode mask there.  Don't know how this
> slipped through review.
> 
> I think your original idea was correct.  Add a new KVM_CAP for the tsc
> deadline timer.  Userspace can add the bit to cpuid if either it
> implements the feature in a userspace apic, or if it finds the new
> capability and uses the kernel apic.

Is it necessary to use KVM_CAP? If I didn't misunderstand, the KVM_CAP sulotion would be:
1. qemu get kvm tsc deadline timer capability by KVM_CAP_...;
2. qemu add cpuid bit
        if ((guest use qemu apic && qemu emualte tsc deadline timer) || 
           (guest use kvm apic && kvm emulate tsc deadline timer (KVM_CAP)))
3. qemu ioctl KVM_SET_CPUID2
4. kvm expose the feature to guest by saving it at vcpu->arch.cpuid_entries, 
seems it's logically redundant.

Jan's patch v2 is a straight forward and simple fix. in the patch
    if (apic) { ... }
means apic (and then its sub-logic tsc deadline timer) emulated by kvm, that's enough:
if quest use kvm apic, it's OK to add cpuid bit and expose to guest;
if guest don't use kvm apic, it will not touch cpuid bit;

Thanks,
Jinsong--
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