linux-kernel - Re: [PATCH] allow userspace to adjust kvmclock offset

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Date:	Tue, 13 Oct 2009 09:28:28 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] allow userspace to adjust kvmclock offset

On Mon, Oct 12, 2009 at 10:53:26AM +0200, Avi Kivity wrote:
> On 10/06/2009 07:24 PM, Glauber Costa wrote:
>> When we migrate a kvm guest that uses pvclock between two hosts, we may
>> suffer a large skew. This is because there can be significant differences
>> between the monotonic clock of the hosts involved. When a new host with
>> a much larger monotonic time starts running the guest, the view of time
>> will be significantly impacted.
>>
>> Situation is much worse when we do the opposite, and migrate to a host with
>> a smaller monotonic clock.
>>
>> This new proposed ioctl will allow userspace to inform us what is the monotonic
>> clock value in the source host, so we can keep the time skew short, and more
>> importantly, never goes backwards.
>>
>>    
>
>> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
>> index f8f8900..0cd5ad8 100644
>> --- a/include/linux/kvm.h
>> +++ b/include/linux/kvm.h
>> @@ -546,6 +546,7 @@ struct kvm_irqfd {
>>   #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
>>   #define KVM_SET_BOOT_CPU_ID        _IO(KVMIO, 0x78)
>>   #define KVM_IOEVENTFD             _IOW(KVMIO, 0x79, struct kvm_ioeventfd)
>> +#define KVM_ADJUST_CLOCK	  _IOW(KVMIO, 0x7a, __u64)
>>    
>
> Please change to a struct with some reserved space.
Ok, can do it.
 
>
> Do we want an absolute or relative adjustment?
What exactly do you mean?
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