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

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Date:	Wed, 14 Dec 2011 10:18:10 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Dor Laor <dlaor@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Eric B Munson <emunson@...bm.net>,
	mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	ryanh@...ux.vnet.ibm.com, aliguori@...ibm.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 Sun, Dec 11, 2011 at 02:40:58PM +0200, Dor Laor wrote:
> >>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?

No.

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