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Re: [Qemu-devel] [PATCH 10/10] migration: [RFC] Use proper types in json


From: Juan Quintela
Subject: Re: [Qemu-devel] [PATCH 10/10] migration: [RFC] Use proper types in json
Date: Wed, 11 Oct 2017 11:41:34 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

"Daniel P. Berrange" <address@hidden> wrote:
> On Tue, Oct 10, 2017 at 08:15:42PM +0200, Juan Quintela wrote:
>> We use int for everything (int64_t), and then we check that value is
>> between 0 and 255.  Change it to the valid types.
>> 
>> For qmp, the only real change is that now max_bandwidth allows to use
>> the k/M/G suffixes.
>
> So on input apps previous would use:
>
>     "max-bandwidth":  1024
>
> ie json numeric field, and would expect to see the same when reading
> data back from QEMU.
>
> With this change they could use a string field
>
>     "max-bandwidth":  "1k"

Actually it is worse than that


if you set:

     "max-bandwidth":  1024

it understand 1024M, and it outputs that big number

      "max-bandwidth":  $((1024*1024*1024))

(no, I don't know the value from memory)

And yes, for migrate_set_parameter, we introduced it on 2.10.  We should
have done the right thing, but  I didn't catch the error (Markus did,
but too late, release were already done)

> As long as QEMU's JSON parser accepts both number & string values
> for the 'size' type it is still backwards compatible if an app
> continues to use 1024 instead of "1k"
>
> On *output* though (ie 'info migrate-parameters') this is not
> compatible for applications, unless QEMU *always* uses the
> numeric format when generating values. ie it must always
> report 1024, and never "1k", as apps won't be expecting a string
> with suffix.  I can't 100% tell whether this is the case or not,
> so CC'ing Markus to confirm if changing "int" to "size" is
> guaranteed back-compat in both directions

This is why I asked.  My *understanding* was that my changes are NOP
if you use the old interface, but I don't claim to be an expert on QAPI.

(qemu) migrate_set_parameter 100
(qemu) info migrate_parameters 
...
max-bandwidth: 104857600 bytes/second
...
(qemu) migrate_set_parameter max-bandwidth 1M
(qemu) info migrate_parameters 
...
max-bandwidth: 1048576 bytes/second
...
(qemu) 

This is the output with my changes applied, so I think that it works
correctly on your example.

Thanks, Juan.



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