Re: [Qemu-devel] [PATCH v1] kvm/x86: Hyper-V tsc page setup

["Denis V. Lunev" <den@openvz.org>]

On 01/20/2016 05:52 PM, Roman Kagan wrote:
> On Wed, Jan 20, 2016 at 05:44:49PM +0300, Denis V. Lunev wrote:
>> On 01/20/2016 05:05 PM, Paolo Bonzini wrote:
>>> On 19/01/2016 08:48, Denis V. Lunev wrote:
>>>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>>>> index 6877b4d7e..93c9e25 100644
>>>>> --- a/include/linux/kvm_host.h
>>>>> +++ b/include/linux/kvm_host.h
>>>>> @@ -145,6 +145,7 @@ static inline bool is_error_page(struct page *page)
>>>>>    #define KVM_REQ_HV_RESET          29
>>>>>    #define KVM_REQ_HV_EXIT           30
>>>>>    #define KVM_REQ_HV_STIMER         31
>>>>> +#define KVM_REQ_HV_TSC_PAGE       32
>>>>>      #define KVM_REQ_MAX               64
>>>> ping
>>> Applied with this change:
>>>
>>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>>> index d7716c6e2752..047c275717d3 100644
>>> --- a/arch/x86/kvm/hyperv.c
>>> +++ b/arch/x86/kvm/hyperv.c
>>> @@ -842,7 +842,7 @@ int kvm_hv_setup_tsc_page(struct kvm_vcpu *vcpu)
>>>   		   tsc_khz, tsc, tsc_scale, tsc_offset);
>>>   	tsc_ref.tsc_sequence++;
>>> -	if (tsc_ref.tsc_sequence == 0)
>>> +	if (tsc_ref.tsc_sequence == 0xFFFFFFFF tsc_ref.tsc_sequence == 0)
>>>   		tsc_ref.tsc_sequence = 1;
>>>   	tsc_ref.tsc_scale = tsc_scale;
>>>
>>> and renumbering KVM_REQ_HV_TSC_PAGE from 32 to 31.
>>>
>>> Paolo
>> the latter does not seem to be correct to me.
>> MS spec has bug, see this thread:
>>     https://lkml.org/lkml/2015/11/2/655
>> and the change was Acked-by: K.Y.
> What happens in the patched code is this: when the sequence counter
> wraps around and hits an invalid value, skip to the next valid one.
>
> I think Paolo wanted to be compatible not only with the de facto
> implementation in Windows Hyper-V guests, but also with the Linux driver
> which followed the buggy spec and thought 0xFFFFFFFF to be invalid.
>
> So the change is fine (as long as there's || rather than a space there
> ;)

this seems reasonable, you are right :)

Thank you for the explanation.