Re: [PATCH v2 0/5] Export offsets of VMCS fields as note information for kdump

[Avi Kivity <avi@redhat.com>]

On 05/21/2012 12:08 PM, Yanfei Zhang wrote:
> 于 2012年05月21日 16:34, Avi Kivity 写道:
> > On 05/21/2012 05:32 AM, Yanfei Zhang wrote:
> >> 于 2012年05月21日 01:43, Avi Kivity 写道:
> >>> On 05/16/2012 10:50 AM, zhangyanfei wrote:
> >>>> This patch set exports offsets of VMCS fields as note information for
> >>>> kdump. We call it VMCSINFO. The purpose of VMCSINFO is to retrieve
> >>>> runtime state of guest machine image, such as registers, in host
> >>>> machine's crash dump as VMCS format. The problem is that VMCS internal
> >>>> is hidden by Intel in its specification. So, we slove this problem
> >>>> by reverse engineering implemented in this patch set. The VMCSINFO
> >>>> is exported via sysfs to kexec-tools just like VMCOREINFO.
> >>>>
> >>>> Here are two usercases for two features that we want.
> >>>>
> >>>> 1) Create guest machine's crash dumpfile from host machine's crash dumpfile
> >>>>
> >>>> In general, we want to use this feature on failure analysis for the system
> >>>> where the processing depends on the communication between host and guest
> >>>> machines to look into the system from both machines's viewpoints.
> >>>>
> >>>> As a concrete situation, consider where there's heartbeat monitoring
> >>>> feature on the guest machine's side, where we need to determine in
> >>>> which machine side the cause of heartbeat stop lies. In our actual
> >>>> experiments, we encountered such situation and we found the cause of
> >>>> the bug was in host's process schedular so guest machine's vcpu stopped
> >>>> for a long time and then led to heartbeat stop.
> >>>>
> >>>> The module that judges heartbeat stop is on guest machine, so we need
> >>>> to debug guest machine's data. But if the cause lies in host machine
> >>>> side, we need to look into host machine's crash dump.
> >>>
> >>> Do you mean, that a heartbeat failure in the guest lead to host panic?
> >>>
> >>> My expectation is that a problem in the guest will cause the guest to
> >>> panic and perhaps produce a dump; the host will remain up.
> >>>
> >>
> >> The point is that before our investigation, we didn't know which side 
> >> leads to this buggy situation. Maybe a bug in host machine or the guest
> >> machine itself causes a heartbeat failure.
> > 
> > How can a guest bug cause a host panic?
> > 
> >> So we want to get both host machine's crash dump and guest machine's
> >> crash dump *at the same time*. Then we could use userspace tools to
> >> get guest machine crash dump from host machine's and analyse them
> >> separately to find which side causes the problem.
> >>
> > 
> > If the guest caused the problem, there would be no panic; therefore
> > there was a host bug.
> > 
>
> Yes, a guest bug cannot cause a host panic. When heartbeat stops in guest
> machine, we could trigger the host dump mechanism to work. This is because
> we want to get the status of both host and guest machine at the same time
> when heartbeat stops in guest machine. Then we can look for bug reasons
> from both host machine's and guest machine's views.

That sounds like a bad idea. Can you explain in what situation it makes
sense for a guest to stop the host (and all other guests running on it)
rather than just restarting the failed services (on the host or other
guests)?

> >>>> Without this feature, we first create guest machine's dump and then
> >>>> create host mahine's, but there's only a short time between two
> >>>> processings, during which it's unlikely that buggy situation remains.
> >>>>
> >>>> So, we think the feature is useful to debug both guest machine's and
> >>>> host machine's sides at the same time, and expect we can make failure
> >>>> analysis efficiently.
> >>>>
> >>>> Of course, we believe this feature is commonly useful on the situation
> >>>> where guest machine doesn't work well due to something of host machine's.
> >>>>
> >>>> 2) Get offsets of VMCS information on the CPU running on the host machine
> >>>>
> >>>> If kdump doesn't work well, then it means we cannot use kvm API to get
> >>>> register values of guest machine and they are still left on its vmcs
> >>>> region. In the case, we use crash dump mechanism running outside of
> >>>> linux kernel, such as sadump, a firmware-based crash dump. Then VMCS
> >>>> information is then necessary.
> >>>
> >>> Shouldn't sadump then expose the VMCS offsets? Perhaps bundling them
> >>> into its dump file?
> >>>
> >>
> >> Firmware-based crash dump doesn't concern the os running on the machine.
> >> So it will not do any os handling when machine crashes.
> > 
> > Seems to me the VMCS offsets are OS independent.
> > 
> Hmm, you mean we could get VMCS offsets in sadump itself?
> But I think if we just export VMCS offsets in kernel, we could use the current
> existing dump tools with no or just very tiny change. I think this could be
> a more general mechanism than making changes in all kinds of dump tools.

The sadump tool generates a core file with the OS image, right? Can it
not attach the offsets to a note, just like you propose for kdump?

-- 
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