linux-kernel - Re: [PATCH 0/4] Export offsets of VMCS fields as note information for kdump

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Date:	Thu, 19 Apr 2012 20:27:07 +0900 (JST)
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	avi@...hat.com
Cc:	zhangyanfei@...fujitsu.com, mtosatti@...hat.com,
	ebiederm@...ssion.com, luto@....edu, joerg.roedel@....com,
	dzickus@...hat.com, paul.gortmaker@...driver.com, gregkh@...e.de,
	ludwig.nussel@...e.de, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH 0/4] Export offsets of VMCS fields as note information
 for kdump

From: Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH 0/4] Export offsets of VMCS fields as note information for kdump
Date: Thu, 19 Apr 2012 13:42:42 +0300

> On 04/19/2012 01:36 PM, HATAYAMA Daisuke wrote:
>> From: Avi Kivity <avi@...hat.com>
>> Subject: Re: [PATCH 0/4] Export offsets of VMCS fields as note information for kdump
>> Date: Wed, 18 Apr 2012 14:56:39 +0300
>>
>> > On 04/18/2012 12:49 PM, zhangyanfei wrote:
>> >> >>
>> >> > 
>> >> > What type of resource?  Can you give an example?
>> >> > 
>> >> Sorry. No concrete example for now.
>> >>
>> >> We are developing this on a conservative policy and I have put the vmcs processing
>> >> in a new module in patch set v2 as you required. The new module is auto-loaded when
>> >> the vmx cpufeature is detected and it depends on module kvm-intel. Loading and unloading
>> >> this module will have no side effect on the running guests.
>> >>
>> >> And one thing I have to stress is that, we can see guest image as crash dump from
>> >> guest machine's view if we have the vmcsinfo, this itself is useful.
>> > 
>> > Why is it useful?  Without a concrete example, it's hard to justify the
>> > code bloat.
>> > 
>>
>> The reason why we want to retrieve guest machine's memory image as
>> crash dump is that then we can debug guest machine's status using
>> symbolic debugger such as gdb and crash utility.
>>
>> This is very useful. Please consider the situation where engineers are
>> forced to look into guest machine's memory image through qemu-kvm's
>> process core dump using gdb without any symbolic information. It's
>> very inefficient.
> 
> I still don't follow.  If qemu crashed, the values in guest registers
> are irrelevant.  In what way can the help debug the qemu crash?
> 

It would be not helpful for the qemu crash case you are concerned
about. We want to use the guest state data to look into guest
machine's image in the crasshed qemu.

Thanks.
HATAYAMA, Daisuke

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