Re: [Qemu-devel] [PATCH v22 25/30] qmp: add x-debug-block-dirty-bitmap-sha256

[John Snow <jsnow@redhat.com>]

On 07/10/2017 02:49 AM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
> 
>> On 07/07/2017 09:53 AM, Markus Armbruster wrote:
>>> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:
>>>
>>>> 07.07.2017 12:00, Markus Armbruster wrote:
>>>>> "Daniel P. Berrange" <berrange@redhat.com> writes:
>>>>>
>>>>>> On Fri, Jul 07, 2017 at 10:05:22AM +0200, Markus Armbruster wrote:
>>>>>>> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:
>>>>>>>
>>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>>>>>>> ---
>>>>>>>>   block/dirty-bitmap.c         |  5 +++++
>>>>>>>>   blockdev.c                   | 25 +++++++++++++++++++++++++
>>>>>>>>   include/block/dirty-bitmap.h |  1 +
>>>>>>>>   include/qemu/hbitmap.h       |  8 ++++++++
>>>>>>>>   qapi/block-core.json         | 27 +++++++++++++++++++++++++++
>>>>>>>>   tests/Makefile.include       |  2 +-
>>>>>>>>   util/hbitmap.c               | 11 +++++++++++
>>>>>>>>   7 files changed, 78 insertions(+), 1 deletion(-)
>>>>>>> [...]
>>>>>>>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>>>>>>>> index 5c42cc7790..6ad8585400 100644
>>>>>>>> --- a/qapi/block-core.json
>>>>>>>> +++ b/qapi/block-core.json
>>>>>>>> @@ -1644,6 +1644,33 @@
>>>>>>>>     'data': 'BlockDirtyBitmap' }
>>>>>>>>     ##
>>>>>>>> +# @BlockDirtyBitmapSha256:
>>>>>>>> +#
>>>>>>>> +# SHA256 hash of dirty bitmap data
>>>>>>>> +#
>>>>>>>> +# @sha256: ASCII representation of SHA256 bitmap hash
>>>>>>> Spell it SHA-256, please.  The member name @sha256 can stay.
>>>>>>>
>>>>>>> SHA-256 is 256 binary bits.  Please specify how they are represented in
>>>>>>> ASCII.  It better be base64 (RFC 4648), because we use that elsewhere.
>>>>>> It is filled later in this patch using qcrypto_hash_digest, so it is just
>>>>>> a hex string representing the hash, not base64. For the latter you can
>>>>>> use qcrypto_hash_base64
>>>>> I got two points:
>>>>>
>>>>> 1. Whatever encoding we use, it needs to be documented.
>>>>>
>>>>> 2. The fewer binary -> ASCII encodings we use, the better.  We already
>>>>> use base64.
>>>>
>>>>
>>>> ASCII format for check sum is more common as it is more readable. It
>>>> is used in the internet to check downloads, it is used by standard
>>>> utility sha256sum. So, it may be better for the monitor.
>>>>
>>>> However, if it is needed, I can make a follow-up patch, it is very
>>>> easy, just s/qcrypto_hash_digest/qcrypto_hash_base64/ in
>>>> util/hbitmap.c. iotest 165 - the only user of the feature - doesn't
>>>> need any changes.
>>>
>>> If the is a standard way to represent SHA-256 in ASCII, use it.
>>>
>>> Whatever you use, document it clearly in the QAPI schema.
>>>
>>
>> I... should we, though? It's a debug interface for testing only,
>> basically. Couldn't think of a better way to test it, and people
>> demanded tests.
>>
>> How's this for documentation:
>>
>> "The hash will be in an arbitrary format that changes every time you
>> look away from this specification. Any similarity, real or imagined, to
>> a canonical SHA-256 ASCII string is purely coincidental."
>>
>> Basically, I would actually rather go out of my way to obfuscate this
>> command, not document it...
>>
>> Maybe that's wrong-headed of me, but I still maintain that it's not
>> terribly important because I'd rather people never, ever try to use this
>> in production.
> 
> It is wrong-headed of you :)
> 

I figured so, but I wanted to be very facetiously clear that we didn't
really intend for this to be used outside of our own testing suite. I'd
love to not even have it be a real QMP interface available in
non-testing binaries at all, but that gets weird quickly.

Sorry for being a wise-ass.

> We mark stuff not covered by QMP's compatibility promise with x-
> prefixes, not with incomplete documentation, and not by obsfuscating it.
> People trying to use this in non-production need to know how it works,
> too.
> 
> If you think people need to be scared away some more, feel free to write
> something like "currently SHA-256 ASCII".  If that's not enough of a
> hint for you, you might add "but you shouldn't rely on that".
> 

I'd still like to decorate it with some little emoji skulls, too. ☠☠💀💀
Make sure people know it's a very bad idea to use it. Maybe QEMU can do
some wall text whenever it gets used that says "Someone is doing
something very naughty when we asked very nicely for them not to."

Kidding again, of course, but just trying to emphasize that this command
literally serves no purpose other than for testing and is not expected
to be used apart from that.

The format could be changed if you desired, but as this has been on the
list for over a year, I'd have to unfortunately request that you make
that change if it was important to you.

--js