Avi Kivity wrote:
> Jan Kiszka wrote:
>>>>
>>>> struct kvm_pit_state {
>>>> struct kvm_pit_channel_state channels[3];
>>>> + u8 hpet_legacy_mode;
>>>> };
>>>>
>>> This changes the ABI, breaking older binaries running on newer kernels,
>>> or newer binaries running on older kernels.
>>>
>>
>> As we have KVM_CREATE_PIT2 now, which includes struct kvm_pit_config
>> with a lot of unused flags, it should be straightforward to negotiate
>> the kvm_pit_state format between kernel and user space: kernel
>> advertises support for the new one via capability, user space requests
>> it via a bit in kvm_pit_state.flags.
>>
>
> We still need a new ioctl. The structure size is embedded in the ioctl
> number, so any additions automatically cause version mismatches.
>
> I sent patches some time ago to have the kernel automatically adjust for
> this, but they weren't well received.
Unfortunate. But on the one hand, nothing technically prevents defining
the IOCTL base on existing kvm_pit_state, but passing down extended
kvm_pit_state2 if that negotiation took place. On the other hand, we are
not yet running out of IOCTL numbers...
However, I guess kvm_pit_state2 will also need some flags field and a
bit tail room for potential future extensions.
Jan
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