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

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On Tue, 30 Aug 2011 15:36:13 -0300, Marcelo Tosatti wrote:
On Tue, Aug 30, 2011 at 10:37:13AM -0600, Eric B Munson wrote:
On Tue, Aug 30, 2011 at 06:26, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> 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
>> --
> How is the host supposed to set this flag?

The hypervisor will set the flag (the second bit in the struct
pvclock_vcpu_time_info.flags field) when it stops the guest. I could
be reading pvclock-abi.h incorrectly, but the way I understand it is
that this structure is used to communicate between hypervisor and

Yes, its awkward though (to have an interface to modify a bit in
pvclock->flags from QEMU).

Anyway, it appears that all elements to fix this in QEMU without the
need for a paravirt interface are available... did you look into my

I have not yet, but will look into it.  I sent these out because I was
under the impression that there was some discussion at LinuxCon about
your suggestion and that it was agreed that the all user space solution
was not feasible.

> 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.

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