Re: [PATCH 5/5] HPET interaction with in-kernel PIT (v6)

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> Beth Kon wrote:
>> Signed-off-by: Beth Kon <eak@us.ibm.com>
>>
>>   
> 
> Please write a few words on what this patch does and why.
> 
>> ---
>>  arch/x86/include/asm/kvm.h |    1 +
>>  arch/x86/kvm/i8254.c       |   24 +++++++++++++++++++-----
>>  arch/x86/kvm/i8254.h       |    3 ++-
>>  arch/x86/kvm/x86.c         |    5 ++++-
>>  4 files changed, 26 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm.h b/arch/x86/include/asm/kvm.h
>> index 708b9c3..3c44923 100644
>> --- a/arch/x86/include/asm/kvm.h
>> +++ b/arch/x86/include/asm/kvm.h
>> @@ -235,6 +235,7 @@ struct kvm_guest_debug_arch {
>>  
>>  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.

Jan


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