Re: [Qemu-devel] [RFC][PATCH 09/15] introduce a new monitor command 'dump' to dump guest's memory

[Eric Blake]

On 01/29/2012 10:36 PM, Wen Congyang wrote:
>>> +++ b/hmp-commands.hx
>>> @@ -828,6 +828,22 @@ new parameters (if specified) once the vm migration 
>>> finished successfully.
>>>  ETEXI
>>>  
>>>      {
>>> +        .name       = "dump",
>>> +        .args_type  = "file:s",
>>> +        .params     = "file",
>>> +        .help       = "dump to file",
>>> +        .user_print = monitor_user_noop,
>>> +        .mhandler.cmd = hmp_dump,
>>> +    },
>>
>> What if I want to dump only a fraction of the memory?  I think you need
>> optional start and length parameters, to limit how much memory to be
>> dumped, rather than forcing me to dump all memory at once.
>>
> 
> It is OK to support it, but I do not know why do you want it?
> 
> The purpose of this command is dumping the memory when the guest is paniced.
> And then we can use crash/gdb(or other application) to investigate why the 
> guest
> is paniced. So we should dump the whole memory.

That's one purpose, but not the only purpose.  We shouldn't be
artificially constraining things into requiring the entire memory region
in order to use this command.

Libvirt provides virDomainMemoryPeek which currently wraps the 'memsave'
and 'pmemsave' monitor commands, but these commands output raw memory.
Your command is introducing a new memory format into ELF images, and if
'memsave' can already do a subset of memory, it also makes sense for
'dump' to do a subset when creating the ELF image.  That is, if a
management app every has a reason to access a subset of memory, then
this reason exists whether the subset is raw or ELF formatted when
presented to the management app.

Meanwhile, on the libvirt side, the virDomainMemoryPeek API to
management apps is constrained - it sends the data inline with the
command, rather than on a side channel.  Someday, I'd like to enhance
libvirt to have a dump-to-stream command, and reuse the existing libvirt
ability to stream large amounts of data on side channels, in order to
let management apps directly and atomically query a subset of memory
into a file with the desired formatting, rather than the current
approach of constraining the management app to only query 64k at a time
and to have to manually pause the guest if they need to atomically
inspect more memory.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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