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

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Date:	Tue, 30 Aug 2011 17:50:32 -0600
From:	emunson@...bm.net
To:	Marcelo Tosatti <mtosatti@...hat.com>
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, 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@...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?

 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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