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

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Message-ID: <20110830141217.GF30100@us.ibm.com>
Date:	Tue, 30 Aug 2011 09:12:17 -0500
From:	Ryan Harper <ryanh@...ibm.com>
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,
	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

* 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?

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@...ibm.com
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