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

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Message-ID: <4AD2EE86.50807@redhat.com>
Date:	Mon, 12 Oct 2009 10:53:26 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] allow userspace to adjust kvmclock offset

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.

Do we want an absolute or relative adjustment?

-- 
error compiling committee.c: too many arguments to function

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