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

On Fri, 09 Sep 2011, Marcelo Tosatti wrote:

On Thu, Sep 01, 2011 at 02:27:49PM -0600, emunson@mgebm.net wrote:

quoted

On Thu, 01 Sep 2011 14:24:12 -0500, Anthony Liguori wrote:

quoted

On 08/30/2011 07:26 AM, Marcelo Tosatti wrote:

quoted

On Mon, Aug 29, 2011 at 05:27:11PM -0600, Eric B Munson wrote:

quoted

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.

Marcelo,

I think that stopping the TSC is the wrong approach because it will break time
between the two systems so timething that expects the monotonic clock to move
consistently will be wrong.  IMO, messing with the TSC at run time to avoid a
watchdog message is the wrong solution, better to teach the watchdog to ignore
this special case.

Thanks,
Eric