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

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Message-ID: <20110830183613.GB23897@amt.cnet>
Date:	Tue, 30 Aug 2011 15:36:13 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Eric B Munson <emunson@...bm.net>
Cc:	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, aliguori@...ibm.com, raharper@...ibm.com,
	kvm-ia64@...r.kernel.org
Subject: Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host

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@...hat.com> 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
> >>
> >> --
> >> 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?

> > 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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