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

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Message-ID: <20091019195128.GC17781@amt.cnet>
Date:	Mon, 19 Oct 2009 17:51:28 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com
Subject: Re: [PATCH] v5: allow userspace to adjust kvmclock offset

On Fri, Oct 16, 2009 at 03:28:36PM -0400, 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.
> 
> [ v2: uses a struct with a padding ]
> [ v3: provide an ioctl to get clock data too ]
> [ v4: used fixed-width signed type for delta ]
> [ v5: nitpick fixes, documentation update, and capability bit ]
> 
> Signed-off-by: Glauber Costa <glommer@...hat.com>

Applied, thanks (the ioctl number changed though).
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