Message-ID: <4FB9A92D.7050108@cn.fujitsu.com>
Date: Mon, 21 May 2012 10:32:13 +0800
From: Yanfei Zhang <zhangyanfei@cn.fujitsu.com>
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To: Avi Kivity <avi@redhat.com>
CC: mtosatti@redhat.com, ebiederm@xmission.com, luto@mit.edu,
        Joerg Roedel <joerg.roedel@amd.com>, dzickus@redhat.com,
        paul.gortmaker@windriver.com, ludwig.nussel@suse.de,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        kexec@lists.infradead.org, Greg KH <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v2 0/5] Export offsets of VMCS fields as note information
 for kdump
References: <4FB35C48.30708@cn.fujitsu.com> <4FB92D5A.3060507@redhat.com>
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?? 2012??05??21?? 01:43, Avi Kivity д??:
> 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.

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.

>> 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.

Thanks
Zhang Yanfei

 

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