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

[emunson]

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. ÂThis set provides the ability for qemu 
> > to comminucate
> > >> to the guest that it has been stopped. ÂWhen the guest hits the 
> > watchdog on
> > >> resume it will check if it was suspended before issuing the 
> > warning.
> > >>
> > >> Eric B Munson (4):
> > >> Â Add flag to indicate that a vm was stopped by the host
> > >> Â Add functions to check if the host has stopped the vm
> > >> Â Add generic stubs for kvm stop check functions
> > >> Â Add check for suspended vm in softlockup detector
> > >>
> > >> Âarch/x86/include/asm/pvclock-abi.h | Â Â1 +
> > >> Âarch/x86/include/asm/pvclock.h   |  Â2 ++
> > >> Âarch/x86/kernel/kvmclock.c     |  14 ++++++++++++++
> > >> Âinclude/asm-generic/pvclock.h   Â|  14 ++++++++++++++
> > >> Âkernel/watchdog.c         Â|  12 ++++++++++++
> > >> Â5 files changed, 43 insertions(+), 0 deletions(-)
> > >> Âcreate mode 100644 include/asm-generic/pvclock.h
> > >>
> > >> --
> > >> 1.7.4.1
> > >
> > > 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
> > guest.
>
> 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
> suggestion?

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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