Re: [PATCH] kvmclock: Ensure time in migration never goes backward

[Alexander Graf <agraf@suse.de>]

On 08.05.14 03:33, Marcelo Tosatti wrote:
> On Mon, May 05, 2014 at 03:51:22PM +0200, Alexander Graf wrote:
>> When we migrate we ask the kernel about its current belief on what the guest
>> time would be. However, I've seen cases where the kvmclock guest structure
>> indicates a time more recent than the kvm returned time.
>>
>> To make sure we never go backwards, calculate what the guest would have seen
>> as time at the point of migration and use that value instead of the kernel
>> returned one when it's more recent.
>>
>> While this doesn't fix the underlying issue that the kernel's view of time
>> is skewed, it allows us to safely migrate guests even from sources that are
>> known broken.
>>
>> Signed-off-by: Alexander Graf <agraf@suse.de>
> OK Alexander better move this logic to the kernel, in KVM_GET_CLOCK.
>
> Otherwise every user of KVM_GET_CLOCK would have to apply the
> workaround.

Well, the breakage occurs on the *source* of things. So if I have 100 
VMs running, I'm pretty sure one of them gets hit by this bug.

If I put the workaround in the kernel, I have to take the chance to 
break some of those 100 VMs to get things rolling. If I put it in QEMU, 
I can live with a broken migration source.

This gets even worse when you want to phase out unfixed host kernels.


Alex