linux-kernel - Re: [PATCH] v5: allow userspace to adjust kvmclock offset

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Date:	Tue, 20 Oct 2009 08:38:51 +0900
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] v5: allow userspace to adjust kvmclock offset

On 10/17/2009 04:28 AM, 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 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. Userspace may also need to trigger
> the current data, since from the first migration onwards, it won't be
> reflected by a simple call to clock_gettime() anymore.
>
> +struct kvm_clock_data {
> +	__u64 clock;
> +	__u64 pad[2];
> +};
> +
>    

The padding is not reusable without a flags word that we can set bits in 
for extended functionality.  Please add one (and a check that it is 
zero, or -EINVAL).

Since it was already applied, please do it as an incremental patch.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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