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Re: [Qemu-devel] hmp interface for kdump compressed format


From: Markus Armbruster
Subject: Re: [Qemu-devel] hmp interface for kdump compressed format
Date: Thu, 27 Mar 2014 09:38:52 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2 (gnu/linux)

"address@hidden" <address@hidden> writes:

> On 03/27/2014 01:04 AM, Markus Armbruster wrote:
>>>>>> So something like adding
>>>>>> >>>>
>>>>>> >>>>  dump_guest_memory_set_format<format>
>>>>>> >>>>
>>>>>> >>>>  would be the only possible solution with the hmp code as
>>>>>> >>>> is. Correct?
>>>>> >>>
>>>>> >>>  Yes, one possibility would be to make the dump command stateful (=
>>>>> >>>  compression format), and to add a new command
>>>>> >>> setting/getting that state.
>>>>> >>>
>>>>> >>>  Another option would be to leave the current command intact,
>>>>> >>> and add an
>>>>> >>>  independent command that wouldn't take "begin", "end", nor
>>>>> >>> "paging", but
>>>>> >>>  would take compression format.
>>>> >>
>>>> >>  Another possibility is to kill begin/length from the
>>>> >>  dump-guest-memory command.
>>>> >>  HMP is not stable, so if that's useful we can do it.
>>> >
>>> >  AFAIK, kdump-compressed format can only be analyzed by crash-utility 
>>> > right
>>> >  now. ELF is big but we still need it, then begin/length will help us save
>>> >  time and space when only part of the memory is need.
>>> >
>>> >  IMHO, a new HMP command will be a good choice and it is more simple than
>>> >  making dump format 'stateful'. I am not so clear about HMP, but I think
>>> >  the new HMP command can be 'dump-guest-memory-with-format', and still
>>> >  base on qmp_dump_guest_memory. If you guys like it, I will send a patch
>>> >  that add this command.
>> Paolo encouraged you to*break*  the existing HMP command instead of
>> adding a new one.  I'd like to second that.
>> .
>>
>
> Hello markus,
>
> I have finish my patch as I stated in my last mail, but something is
> wrong with my mail server, I have to wait a couple of days before
> sending the patch. You said you prefer *breaking* the existing HMP
> command. Since paging/begin/length is still usefull, would you please
> give some reason and I will consider that.

I'm encouraging you to design a HMP command that gives you all you need.
Then call that dump-guest-memory, backward compatibility be damned.

Would

    dump-guest-memory [-p] filename [[format] begin length]

do?

An alternative could be two commands:

    dump-guest-memory filename [format [begin length]]
    dump-guest-memory-elf [-p] filename [begin length]

This bakes the restriction "paging, begin and length work only with
format 'elf'" into the interface.  Not sure that's a good idea, but I'm
leaving that to folks who actually know something about this dumping
business.

If you can find better command names, go right ahead.  You could pick
names that avoid incompatible change.  I care about avoiding
incompatible change to exotic HMP commands even less (a lot less, in
fact) than about naming of exotic HMP commands.



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