Re: [Qemu-devel] [PATCH v5 09/11] pci bridge dev: change msi property type

[Cao jin <caoj.fnst@cn.fujitsu.com>]

On 05/15/2016 09:25 PM, Marcel Apfelbaum wrote:
> On 05/06/2016 07:20 AM, Cao jin wrote:
>>  From bit to enum OnOffAuto.
>>
>> cc: Michael S. Tsirkin <mst@redhat.com>
>> cc: Markus Armbruster <armbru@redhat.com>
>> cc: Marcel Apfelbaum <marcel@redhat.com>
>> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
>> ---
>>
>> Actually, I am not quite sure this device need this change, RFC.
>>
>
> Well, it already has the 'msi' property, so we may want to make it
> standard 'OnOffAuto'.
> One problem I can see is the change of semantics. Until now msi=on means
> 'auto'. From now on
> it means 'force msi=on', fail otherwise. If I got this right,  old
> machines having msi=on
> will failed to start on platforms with msibroken=true, right?

Exactly, and patch 11 indeed has semantics change. According to what we 
discussed before: "if user want msi=on explicitly on command line, then 
msi_init`s failure should result in failing to create device", this 
semantics change seems can`t be avoid.

> Maybe we should preserve the semantics for old machines? (this patch
> does not actually
> affect the semantics, but patch 11/11 should, otherwise why change it to
> OnOffAuto, right? )

IMHO, in this case, keep compat maybe a burden on us, and make 
command-line confusing. See my understanding:

before:
command line format is msi=on/off, and 'on' means auto

now:
we change command line to msi=on/off/auto, and keep compat is meaning 
'on' still means auto?  If so, why need 'auto'

So, I am thinking, maybe we can help old machine user update their 
configuration, I mean: when they set msi=on on platforms with 
msibroken=true, they definitely fail, so we should give a clear hint in 
error message to help them know what should do to start the machine, 
like "please use msi=auto and try again"

-- 
Yours Sincerely,

Cao jin