Re: [PATCH 3/3] QEMU kvm/i386 : Adding KICK_VCPU capability support in i386 target.

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-12-19 15:03, Alexander Graf wrote:
> 
> On 19.12.2011, at 14:59, Avi Kivity wrote:
> 
>> On 12/19/2011 03:54 PM, Alexander Graf wrote:
>>> On 04.12.2011, at 19:26, Raghavendra K T wrote:
>>>
>>>> Extend the KVM Hypervisor to enable KICK_VCPU feature that allows
>>>> a vcpu to kick the halted vcpu to continue with execution in PV ticket
>>>> spinlock.
>>>>
>>>> Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
>>>> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>>>> ---
>>>> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
>>>> index 5bfc21f..69bce21 100644
>>>> --- a/target-i386/kvm.c
>>>> +++ b/target-i386/kvm.c
>>>> @@ -97,6 +97,7 @@ struct kvm_para_features {
>>>>    { KVM_CAP_NOP_IO_DELAY, KVM_FEATURE_NOP_IO_DELAY },
>>>>    { KVM_CAP_PV_MMU, KVM_FEATURE_MMU_OP },
>>>>    { KVM_CAP_ASYNC_PF, KVM_FEATURE_ASYNC_PF },
>>>> +    { KVM_CAP_KICK_VCPU, KVM_FEATURE_KICK_VCPU },
>>>
>>> So this is handled in the kernel? Who enables the feature? Is it always on? Why bother with it in user space at all then?
>>
>> Backwards compatibility
> 
> If we want backwards compatibility, we need more than just a simple feature check, no? Oh, you feed that into CPUID? That's nifty. Ok, so you behave like VMX/SVM do on real hardware - you always expose the functionality but don't list it in CPUID for older user space.

Do we want this to be on when providing a compat machine type ("pc-0.12"
etc.) to the guest? Then it does need more work (see the dance around
kvmclock).

Jan

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