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

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Date:	Tue, 30 Aug 2011 11:35:51 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Ryan Harper <ryanh@...ibm.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,
	Anthony Liguori <aliguori@...ibm.com>,
	Ryan A Harper <raharper@...ibm.com>, kvm-ia64@...r.kernel.org
Subject: Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host

On Tue, Aug 30, 2011 at 09:12:17AM -0500, Ryan Harper wrote:
> * Marcelo Tosatti <mtosatti@...hat.com> [2011-08-30 07:35]:
> > 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.
> 
> How is saving the offset going to prevent a large jump from triggering
> the softlock message?  Won't we still have not touched the watchdog for
> that long period of time?

The idea is to adjust the offset and the TSC value so that the guest
does not notice the actual elapsed time. This is what happens when a
guest is migrated or saved/restore, for example.

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