linux-kernel - Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host

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Message-ID: <4EE4A4DA.90100@redhat.com>
Date:	Sun, 11 Dec 2011 14:40:58 +0200
From:	Dor Laor <dlaor@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Eric B Munson <emunson@...bm.net>, mingo@...hat.com, hpa@...or.com,
	arnd@...db.de, ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com,
	mtosatti@...hat.com, jeremy.fitzhardinge@...rix.com,
	levinsasha928@...il.com, Jan Kiszka <jan.kiszka@...mens.com>,
	kvm@...r.kernel.org, linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped
 by host

On 12/07/2011 04:41 PM, Avi Kivity wrote:
> On 12/05/2011 10:18 PM, Eric B Munson wrote:
>> Changes from V4:
>> Rename KVM_GUEST_PAUSED to KVMCLOCK_GUEST_PAUSED
>> Add description of KVMCLOCK_GUEST_PAUSED ioctl to api.txt
>>
>> Changes from V3:
>> Include CC's on patch 3
>> Drop clear flag ioctl and have the watchdog clear the flag when it is reset
>>
>> Changes from V2:
>> A new kvm functions defined in kvm_para.h, the only change to pvclock is the
>> initial flag definition
>>
>> Changes from V1:
>> (Thanks Marcelo)
>> Host code has all been moved to arch/x86/kvm/x86.c
>> KVM_PAUSE_GUEST was renamed to KVM_GUEST_PAUSED
>>
>> When a guest kernel is stopped by the host hypervisor it can look like a soft
>> lockup to the guest kernel.  This false warning can mask later soft lockup
>> warnings which may be real.  This patch series adds a method for a host
>> hypervisor to communicate to a guest kernel that it is being stopped.  The
>> final patch in the series has the watchdog check this flag when it goes to
>> issue a soft lockup warning and skip the warning if the guest knows it was
>> stopped.
>>
>> It was attempted to solve this in Qemu, but the side effects of saving and
>> restoring the clock and tsc for each vcpu put the wall clock of the guest behind
>> by the amount of time of the pause.  This forces a guest to have ntp running
>> in order to keep the wall clock accurate.

Guests need to run NTP regardless, not only the virtualization layer add 
some skew, the physical world is not that perfect.
btw: traditional NTP client won't sync the time automatically if the 
diff is > 0.5%.

>
> Having this controlled from userspace means it doesn't work for SIGSTOP
> or for long scheduling delays.  What about doing this automatically
> based on preempt notifiers?
>
>

Isn't it solved by steal time?
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