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List: kvm-ia64
Subject: Re: [PATCH 0/4] Avoid soft lockup message when KVM is stopped by host
From: emunson () mgebm ! net
Date: 2011-08-30 23:50:32
Message-ID: 7a46c2b8bc6602df7f3e48a92d826c2e () mgebm ! net
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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@redhat.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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