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

* Marcelo Tosatti [off-list ref] [2011-08-30 07:35]:

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.

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@us.ibm.com