linux-kernel - Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host

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Date:	Thu, 01 Sep 2011 14:24:12 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Eric B Munson <emunson@...bm.net>, avi@...hat.com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	riel@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, kvm-ppc@...r.kernel.org,
	aliguori@...ibm.com, raharper@...ibm.com, kvm-ia64@...r.kernel.org,
	Glauber Costa <glommer@...il.com>
Subject: Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by
 host

On 08/30/2011 07:26 AM, Marcelo Tosatti wrote:
> On Mon, Aug 29, 2011 at 05:27:11PM -0600, Eric B Munson wrote:
>> Currently, when qemu stops a guest kernel that guest will issue a soft lockup
>> message when it resumes.  This set provides the ability for qemu to comminucate
>> to the guest that it has been stopped.  When the guest hits the watchdog on
>> resume it will check if it was suspended before issuing the warning.
>>
>> Eric B Munson (4):
>>    Add flag to indicate that a vm was stopped by the host
>>    Add functions to check if the host has stopped the vm
>>    Add generic stubs for kvm stop check functions
>>    Add check for suspended vm in softlockup detector
>>
>>   arch/x86/include/asm/pvclock-abi.h |    1 +
>>   arch/x86/include/asm/pvclock.h     |    2 ++
>>   arch/x86/kernel/kvmclock.c         |   14 ++++++++++++++
>>   include/asm-generic/pvclock.h      |   14 ++++++++++++++
>>   kernel/watchdog.c                  |   12 ++++++++++++
>>   5 files changed, 43 insertions(+), 0 deletions(-)
>>   create mode 100644 include/asm-generic/pvclock.h
>>
>> --
>> 1.7.4.1
>
> How is the host supposed to set this flag?
>
> As mentioned previously, if you save save/restore the offset added to
> kvmclock on stop/cont (and the TSC MSR, forgot to mention that), no
> paravirt infrastructure is required. Which means the issue is also fixed
> for older guests.

IIRC, the steal time patches have some logic that basically say:

if there was steal time:
    kick soft lockup detector

I wonder if that serves this purpose provided that time spent in stop is 
accounted as steal time.  If it isn't, perhaps it should be?

Regards,

Anthony Liguori

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